Light modulating process

ABSTRACT

An electrochromic light-modulating process, by reflection or transmission, in which an electrolytic material comprises a mixture of solid consistency of a (a) water-soluble salt of a metal which can be cathodically deposited from an aqueous solution of one of its ions, (b) an initially water-soluble film-forming polymer resin, and (c) water. A layer of electrolytic material is produced. A first transparent electrode is arranged in contact with a first face of the layer. A second electrode is arranged in contact with a second face of the layer, thus forming an elementary light-modulating cell. During a write phase of a certain duration, there is applied to the working electrode an electrical voltage which is negative with respect to that of the second electrode, such that, during this write phase, there is written at least one picture element. During at least one erase phase, subsequent to a write phase, there is made to flow between the electrodes an electrical current whose direction is opposite to that of the electrical current during the write phase such that, during this erase phase, the previously written picture element is erased. The process may be repetitive, and is able to comprise several pairs of one write phase and one erase phase.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. Pat. application Ser.No. 07/221,539, filed July 19, 1988, now abandoned in favor of thepresent application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a light-modulating process, in particular forvariable reflection of light, variable transmission of light, anddisplay of signals and images such as alphanumerical and graphicalinformation and other optical information. The invention findsapplication in various electro-optical devices, in particular displaypanels, screens and devices, and to variable transparency windows, shopwindows, screens, windscreens, spectacles, light valves, shutters,variable reflection mirrors, memories, and so forth.

2. Related Art

Numerous processes and devices for light modulation are known. Amongthose of particular interest are those enabling the production ofelectro-optical devices which are very thin with respect to their area,in particular with regard to display devices and in particular flatscreens of so-called liquid crystals, electrochromic and electrophoretictype.

Among these various light-modulating techniques, the electrochromicprocesses use the reversible change of color and/or of optical densityobtained by the electro-chemical oxidoreduction of a so-calledelectrochromic material whose oxidized form and whose reduced form havedifferent colors and/or optical densities.

Electrochromic light-modulating processes have characteristics which arenoteworthy for numerous applications: low control voltage (having amaximum of a few volts); low energy consumption; open circuit(nonvolatile) memory; and relatively uncritical distance requirementsbetween electrode and counter-electrode. They also have othercharacteristics which are particularly advantageous for display devices:very high contrast even when viewed laterally at a high angle; excellentvisibility by reflection under high-illumination conditions such as inbright sunshine; extended grey scale; and wide operating temperaturerange (often extending to low temperatures).

The low control voltage enables the use of low cost electronic controland addressing means. Furthermore, low energy consumption enablesapplications where independent operation (on batteries or accumulators)is required.

However, known processes and electrochromic devices have a certainnumber of disadvantages which limit their fields of application.

In general, elementary cells of known electrochromic light modulatingdevices are sealed (individually or in combination with other cells) ina way which is strictly leaktight with respect to the external ambientatmosphere. Known cells generally comprise (1) a transparent frontelectrode deposited on (2) a transparent plate of glass or plasticmaterial, (3) an electrochromic material (often in the form of a thinlayer deposited on the transparent electrode), (4) a gap, filled withelectrolyte, (5) a counter-electrode (also transparent if the devicefunctions by transmission), and (6) conductors for electrical connectionof each electrode to an electronic control means external to the cell.Known cells also most often comprise a specific separator intended tomaintain between the electrode and the counter-electrode anelectrolyte-filled gap of constant thickness. Known cells also comprisestructural means employing material and seals intended to maintaincohesion and permanence of internal physical and electrical contactswhich are necessary for correct operation. At least the front electrodeand/or the layer of electrochromic material are delimited in such a wayas to define the shape required for the corresponding picture element(such as image point or image segment).

A strictly leak-tight sealing is necessary to prevent loss (particularlyby leakage or evaporation) of constituents of the internal medium,particularly constituents of the electrolyte. Leak-tight sealing is alsonecessary to prevent the entry into the cell of constituents of externalambient atmosphere (for example, oxygen, carbon dioxide, humidity, andvarious contaminating substances) which are often capable, even intraces, of altering or degrading the constituents of the internalmedium, of introducing parasitic processes, of affecting the operationof the cell, and of reducing its lifetime.

The sealing problem is a significant problem at points where the cellmust provide a sealed passage for the conductors connecting the frontelectrode and the counter-electrode to the external electronic means.The seals, which must be compatible with the various materials used, aresubject to mechanical stresses resulting in particular from differencesbetween the coefficients of expansion of these materials.

This sealing problem is aggravated when the dimensions of the device areincreased. Stresses of thermal origin can increase because of asymmetryin exposure to heat sources. Stresses of mechanical origin occur, due tovibrations to which a panel of large dimensions is naturally exposed.Interaction with the structure for mounting and holding the panel alsointroduce stresses.

The necessity of such a strictly leak-tight sealing, and the problemswhich it raises, are explicitly mentioned and justified by numerousdocuments, with respect to electrochromic materials, electrolytes andvarious structures. In particular, reference is made to U.S. Pat. No.4,127,853; FR 83,041,162 (cell containing a metallic oxide as anelectrochromic material and a liquid organic electrolyte from which themolecular oxygen must be removed); FR 7,443,548 (for several classes ofsolid electrolytes, necessary support--using a sealed casing--ofparticular conditions of humidity, pressure, vacuum or gaseousatmosphere essential for the correct operation of the device); U.S. Pat.No. 4,128,315 (sealing necessary to prevent loss of humidity); U.S. Pat.No. 4,116,546 (use of a solid electrolyte for the particular purpose ofavoiding rapid degradation of the seal observed with liquid or acidicsemi-solid electrolytes); U.S. Pat. No. 4,167,309 (protection fromatmospheric oxygen of radical type electrochromic materials); U.S. Pat.No. 3,704,057 (seal for sealing a cell containing tungsten trioxide asan electrochromic material and a semi-solid gelled electrolyte); U.S.Pat. No. 3,708,220 (cell preventing any leakage by self-sealing of theelectrolyte inlet orifice); J. Duchene et al, IEEE Transactions onElectron Devices, Vol. RD-26, No. 8, August 1986, p. 1263(electro-deposited cell with organic liquid electrolyte sealed by asealing glass).

In known electrochromic processes and devices, there are several typesof electrochromic materials and generation erasure mechanisms of opticaldensity and/or of coloring, each having its own problems which add tothe problems described above. These problems include the following:

1) Oxidoreduction of non-stoechiometric electrochromic solids. Aconsiderable number of electrochromic solids have been used, which aregenerally solids which are insoluble in the two states of oxidationbetween which they change color; these solids are electricallyinsulating or slightly conducting. Among inorganic materials thefollowing can be particularly mentioned among others: WO₃, MoO₃, V₂ O₅,Nb₂ O₅, IrO_(x). (An extensive list is given, for example, in U.S. Pat.No. 3,704,057.) Among organic materials are diphthalocyanine of Lu, andof Yb in particular.

These electrochromic solids must generally be used by depositing a thinlayer on the transparent electrode by means of costly vacuum depositiontechniques (evaporation under vacuum, cathodic sputtering inparticular). Their change of color is generally from colorless or from aprimary color to a second different color: colorless to blue for WO₃ andMoO₃, yellow to green for V₂ O₅, colorless to blue or blue-black forIrO_(x), green to red for diphthalocyanin of lutecium.

The most-used of these electrochromic solids, tungsten trioxide WO₃, hasproblems, in addition to those already mentioned, which arerepresentative of those of this class of electrochromic materials: veryhigh sensitivity to contaminating substances, particularly atmospheric(document FR 83,041,162), degradation by corrosion with dissolution inthe aqueous and polymeric electrolytes (U.S. Pat. No. 4,215,915, U.S.Pat. No. 3,970,365), reduced but not eliminated inorganic electrolytes(Kodintsev et al., Electrokhimiya 1983, Vol. 19, No 9, page 1137).

Complex techniques, for example oblique evaporation (U.S. Pat. No.4,390,246), are required for improving the color generation and erasurecharacteristics which are very sensitive to slight changes inpreparation and composition. In most display devices (for example, U.S.Pat. No. 4,128,315), the tungsten trioxide film must be deposited with adelimitation according to the shape and dimensions of the pictureelement (image segment or image point). Finally, the cells have neitherthe voltage threshold nor the memory in a circuit coupled to other cellswhich would be necessary for multiplexed matrix operation (Yoshiro Mori,J.E.E., August 1985, page 53).

2) Oxidoreduction of radical compounds. The most representative and moststudied of the materials of this class is heptyl-viologen. Dissolved inthe electrolyte where it is colorless, heptyl-viologen deposited byreduction is a blue or red colored film on the transparent electrode andis redissolved by oxidation (U.S. Pat. No. 4,116,535). But it is knownthat the deposit progressively recrystallizes in a form which cannot beredissolved, which severely limits the number of accessible cycles andthe lifetime. Alternatively, the electrode passivates, considerablyreducing the speed of the writing reaction for which it is thennecessary to catalyze, for example, by depositing metallic ions(document EP 0,083,668). Finally, the cells do not have either athreshold or a memory in a circuit coupled to other cells.

3) Electrodeposition of metals. The reversible electrodeposition ofmetals from an electrolytic solution has been the subject of variousworks, particularly with liquid organic electrolytes, because ofcorrosion problems and parasitic reactions harmful to the stability andlifetime encountered with aqueous electrolytes. For example, Y. Ducheneet al. (in the article cited above), describes a display cell which usesas an electrolyte, methanol or acetonitrile containing silver iodide andsodium iodide. The silver ions reduce into a silver film having a highcontrast. However, for a given electrical charge, the optical densitydepends on the current density used, and inhomogeneities appear on thedeposited film after a certain number of deposition redissolutioncycles. The cell does not have a writing voltage threshold and is nottherefore suitable for multiplexed matrix writing. The zone of thetransparent electrode corresponding to the display must be delimitedinside the cell by means of an insulating layer engraved according tothe design of the zone in question. Finally, the use of a glass sealingtechnology is indicated as one of the conditions of reliability,confirming the importance of strictly leak-tight sealing.

A similar cell described by I. Camlibel et al. (Appl. Phys. Letters33,9, Nov. 78, page 793) contains silver iodide and potassium iodide indimethylsulphoxide, and produces a specular gilt or bright red deposit,depending on conditions.

4) Electro-active polymers (redox). Recent works relate to polymers suchas polyaniline, polyacetylene, polyrrole, and polythiophene, inparticular which, in thin layer on a transparent electrode, can changecolor (for example from red to blue for polythiophene] depending ontheir state of oxidation. These materials, which are generally ratherunstable or easily alterable, have a short lifetime and do not enable avery large number of operating cycles.

It has been seen that most known electrochromic cells do not have adefinite electrical voltage threshold (i.e., an electrical voltage belowwhich a picture element is not written). Furthermore, although most ofthese cells have an open circuit (nonvolatile) memory (i.e., apersistence of the written state when the electrical writing voltage isdisconnected), this memory partially discharges if a written 10 cell isconnected to an erased cell, such that the first cell partially eraseswhile the second partially writes. In this event, the optical density ofthe cells tends to become uniform with time. The absence of a definitewriting threshold and/or a persistent memory in a circuit coupled toanother erased cell, prohibit the matrix writing of a system of pictureelements placed at the intersections of two orthogonal arrays ofparallel conductors.

Analysis confirms that the non-selected picture elements are partiallywritten while the selected picture elements are partially erased. Theoptical density of the selected picture elements and that of thenonselected picture elements approach each other, thus degradingcontrast and even eliminating it.

In known systems, it is exceptional to obtain a genuine black in thewritten state. It is also uncommon to obtain a genuinely white orcolorless transparent appearance in the erased state. Generally, colorssuch as blue, blue-black, purple, and so forth, are obtained. Apart fromthe aesthetic preference for a color or for black, the production of aparticular color prohibits a multi-color display by a three-colorprocess (unless it becomes possible to generate the three primarycolors). On the other hand, the production of a genuine black in thewritten state and a genuine white in the erased state (or a colorlesstransparent appearance in transmission) enables multicolor display byadditive synthesis by associating picture elements with blue, green andred colored screens or filters according to a repetitive distribution.

Numerous known electrochromic devices use a liquid electrolyte (forexample, an aqueous electrolyte such as an aqueous solution of sulfuricacid (document FR 7,626,282), or an organic electrolyte such as asolution of lithium perchlorate in propylene carbonate (Yoshiro Moriarticle, cited above)). This electrolyte, which cannot generally becommon to several cells for electrical reasons, requires individualconfinement in each cell which must comprise an electrolytic compartmentwhich must not be distorted. In addition to the problems raised by theindividual filling and sealing of each cell, the particularly complexstructure which is obtained, despite its cost, does not enable a highresolution display device (such as a computer screen). If it appearspossible to reduce the size of the picture element to the necessaryvalues (of the order of a few hundred microns), the size of the cell(and particularly the needed lateral walls), does not enable reductionof the gap between adjacent image-points to a value which should be ofthe order of a few tens of microns at most.

In order to reduce the complexity of the display cell brought about bythe problems of confinement of liquid electrolyte, there has been usedgelled semisolid liquid electrolytes (U.S. Pat. No. 3,708,220: gelledsulfuric acid), polymers with acidic functions (U.S. Pat. No.4,116,545), and ion exchange membranes (U.S. Pat. No. 4,128,315). Thestructure of the cells is actually simplified, and in certain cases hasthe additional advantage of surface adhesion properties (tackiness),simplified construction, and viscoelastic properties which improve thecontacts. But all of these electrolytes used in association with a layerof solid electrochromic material deposited on the transparent electrodecontain, in one way or another, a certain quantity of water (byconstitution, hydration, impregnation, and so forth). The cells have, tovarying degrees, the corrosion problems mentioned above, as well as thenecessity of a leak-tight sealing.

In view of avoiding the use of a free liquid electrolyte, inorganicsolids have also been used which have ionic conductivity, such as forexample beta alumina (M. Green et al., Solid State Ionics 3/4, 1981,pages 141 to 147, North-Holland), or polymers having ionic conductionsuch as, for example, solid solutions of lithium perchlorate inpolyethylene oxide (document FR 8,309,886). However, it is well knownthat such solid electrolytes, at ambient or ordinary temperatures haveonly a generally very low ionic conductivity, considerably impeding thespeed of writing and erasure which may require several seconds or evenmore. Furthermore, a progressive degradation of the electrical contactbetween the inorganic solid electrolytes and the electrodes is oftenobserved. This degradation has a harmful effect on the lifetime of thelight-modulating cells.

In known electrochromic devices, the counter-electrode is often ofcomplex and expensive manufacture and structure because of the functionsthat it may have to simultaneously provide. The functions include theauxiliary redox function, maintaining a constant specified electrodepotential, high charge capacity, reversibility, and so forth, whilebeing capable of a high number of cycles without degradation. Forexample, a counter-electrode has been produced comprising a second layerof an electrochromic solid modified in such a way as to have a lowelectrochromicity and deposited on a transparent electrode (U.S. Pat.No. 4,278,329). Another known counter electrode is a sheet of paperformed with acrylic fibers, a binder and carbon powder, in which thereis also incorporated an electrochromic solid (U.S. Pat. No. 4,088,395).Another counter-electrode whose electrode potential is adjustablecomprises carbon powder, a binder and mixtures of depolarizers W₁₈ O₄₉and V₆ O₁₃ in adjustable proportions (Yoshiro Mori article cited above).

The structure and manufacture of known electrochromic display screensare generally complex and expensive, especially when the size of thepanel is large. Beyond a certain size, technical problems andmanufacturing costs become such that large display panels can only beproduced in the form of a mosaic of small independent panels.

There is also known (in document FR 2,504,290) a process for recordingsignals and images in which:

1) A recording medium is formed, comprising at least an electrochromicmaterial having at least one free surface constituted by a mixture ofsolid consistency of at least (a) a water-soluble salt or awater-soluble mixture of salts of at least one metal which can becathodically deposited from an aqueous solution of one of its ions; and(b) an initially water-soluble film-forming polymer resin, preferably inthe proportion of 1 part by weight to 0.5 to 50 parts of anhydroussalts; and (c) water;

2) There is placed in contact with the free surface of theelectrochromic material, at a place where it is desired to form a mark,an electrode, taken, with respect to the said material, to a negativepotential in order to make an electrical current flow between theelectrode and the material;

3) There is formed in the electrochromic material, in the zone ofcontact with the electrode, a mark which is immediately and directlyvisible by cathodic reduction of at least one depositable metallic ionand is present in the material, and at least one metal whichelectrocrystallizes and is an integral part of the material, the metalconstituting the mark.

According to this document, the electrochromic material, and the processfor its implementation, intend to obtain a mark (signal or image) whichis essentially stable in time.

On the other hand, this document is not interested in, does not suggestand does not describe the application of such an electrochromic materialfor modulating light nor a corresponding implementation process.

SUMMARY OF THE INVENTION

The present invention relates to a light-modulating process avoiding thementioned disadvantages of the processes of known systems.

An object of the invention is to provide an electrochromic process oflight modulation (by reflection and/or by transmission), andparticularly a display process, enabling a reversible and repetitiveuniform increase (by the passage of current in one direction) in thedensity of a common area zone having a clear shape according to acontinuous grey scale up to black or up to opacity, and then to erase orcancel the created optical density.

The process may be achieved by means of a voltage of the order of a fewvolts applied between two electrodes, at least one of which istransparent (and the second also transparent in the case of functioningby transmission). An electrolytic material is located in a gap betweenthese electrodes.

This process furthermore has other noteworthy characteristics which canbe used in isolation or in combination. In particular, thecharacteristics include a writing voltage threshold, nonvolatility (or"memory") of the created optical density, maintenance of high contrasteven when viewed at lateral angles, and high resolution.

For this purpose, the invention provides an electrochromic process formodulating light by reflection or transmission, comprising the followingsteps:

1) Forming at least one electrolytic material comprising a mixture ofsolid consistency of at least (a) a water-soluble salt or a watersoluble mixture of salts of at least one metal which can be cathodicallydeposited from an aqueous solution of one of its simple or complex ions;(b) at least one initially water-soluble film-forming polymer resin,preferably in the proportion of one part by weight to 0.05 to 50 partsof anhydrous salts; and (c) water;

2) Producing at least one layer of at least one electrolytic materialhaving a thickness of between a few microns and a few tens of microns;

3) Arranging a first transparent or substantially transparent electrode(here denoted a "working electrode") in contact with a first face of thelayer of electrolytic material;

4) Arranging a second electrode (here denoted a "counter-electrode") incontact with a second face of the layer of electrolytic material;

steps 1-4 thus forming an elementary light-modulating cell;

5) Applying to the working electrode, during at least one write phase ofa certain duration, an electrical voltage which is negative with respectto that of the counterelectrode, such that, during this write phase,there is written at least one picture element (image point or imagesegment). That is, an increase in optical density is obtained in theinterface region between the working electrode and the layer ofelectrolytic material; and

6) Causing to flow between the electrodes (during at least one erasephase subsequent to a write phase) an electrical current whose directionis opposite to that of the electrical current during the write phase,such that, during this erase phase, the previously written pictureelement (image point or image segment) is erased; that is, thepreviously obtained increase in optical density is diminished ordisappears.

The process is repetitive, and may comprise several pairs of one writephase and one erase phase.

According to other characteristics which will result from what follows,this process also has, optionally, alone or in combination, thefollowing characteristics.

The process may comprise a step for maintaining the written pictureelement for a certain duration following a write phase, during which noexternal potential difference is applied between the working electrodeand the counterelectrode. The optical densification produced during thewrite phase remains, at least in part, at least for a certain timeduring the maintaining phase.

The process may comprise a step for maintaining the written pictureelement for a certain duration following a write phase, during which isapplied a write voltage close to the electro-motive force of theelementary cell in the written state.

An optical densification of a picture element is produced which canhave, in a continuous way, any intermediate value between the opticaldensity in the erased state and the maximum density in the writtenstate. For this purpose, there is made to flow between the electrodes anelectrical charge smaller than that which is necessary for obtaining themaximum optical density.

There is applied between the two electrodes a voltage difference atleast equal to a defined electrical voltage threshold, below which thepicture element is not written.

During at least part of an erase phase, there is applied to theelectrodes an electrical potential difference of direction opposite tothat applied during the previous write phase.

During at least part of an erase phase, an electrical short circuit isproduced between the two electrodes.

During a write phase, a metallic image is developed by cathodicreduction of metallic ions; and, during an erase phase, a metallic imageis erased by anodic oxidation.

The invention enables, as described below, the overcoming of thedisadvantages of known light-modulating techniques. The inventionenables the writing, erasing and maintaining, in written or erasedstate, in a reversible and repetitive way, a picture element, which isnot foreseen in the document FR 2,504,290.

Other characteristics of the invention will be understood with the helpof the accompanying drawings and the following Detailed Description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 2A, 2B, 3A, 3B, 4A, and 4B are four pairs of drawingsshowing for each pair a top plan view and a side elevational view of anelementary display cell. FIGS. 1A and 1B show the cell at rest. FIGS. 2Aand 2B show the cell during a write phase. FIGS. 3A and 3B show the cellin a maintaining phase. FIGS. 4A and 4B show the cell in an erase phase.

FIG. 5 is an elevation view showing the writing of picture elements bycombined superimposed patterns of the electrode, counter-electrode andelectrolytic material.

FIG. 6 is a cross-sectional view showing on embodiment of the layer ofthe present electrolytic material.

FIG. 7A is a cross-sectional view showing a second embodiment of a cell.

FIG. 7B is a cross-sectional view showing a third embodiment of a cell.

FIG. 8A is a cross-sectional view showing a first embodiment of amodulating cell.

FIG. 8B is a cross-sectional view showing a second embodiment of amodulating cell.

FIG. 8C is a cross-sectional view showing a third embodiment of amodulating cell.

FIG. 8D is a cross-sectional view showing a fourth embodiment of amodulating cell.

FIG. 8E is a cross-sectional view showing a fifth embodiment of amodulating cell.

FIG. 8F is a cross-sectional view showing a sixth embodiment of amodulating cell.

FIG. 8G is a cross-sectional view showing a seventh embodiment of amodulating cell.

FIG. 8H is a cross-sectional view showing an eighth embodiment of amodulating cell.

FIG. 8I is a cross-sectional view showing a ninth embodiment of amodulating cell.

FIG. 9 is a top plan view of a tenth embodiment of a device constructedusing strip material for the component parts.

FIG. 10 is a cross-sectional view corresponding to the embodiment ofExample 1 (in the Detailed Description).

FIG. 11 is an elevation view corresponding to the embodiment of Example4.

FIG. 12 is a cross-section view taken along the line XII--XII of FIG.11.

FIG. 13 is a cross-section view corresponding to the embodiment ofExample 6.

FIG. 14 is an elevation view corresponding to the embodiment of Example7.

FIG. 15 is a cross-section view taken along the line XV--XV of FIG. 14.

FIGS. 16 is a front elevation view of a display panel with directaddressing corresponding to the embodiment of Example 11.

FIGS. 17 is a rear elevation view of a display panel with directaddressing corresponding to the embodiment of Example 11.

FIGS. 18, 19 and 20 are partial cross-sectional views of the embodimentof FIGS. 16 and 17.

FIG. 21 is a detailed partial cross-section view of an alternativeembodiment for electrical connection.

FIGS. 22 and 23 are front elevation views of a panel according to theembodiment of Example 12.

FIG. 24 is a front elevation of a panel according to the embodiment ofExample 13.

FIG. 25 is a rear elevation of a panel according to the embodiment ofExample 13.

FIG. 26 is a cross-sectional view taken along the line XXVI--XXVI ofFIG. 24.

FIG. 27 is a cross-sectional view taken along the line XXVII--XXVII ofFIG. 24.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Definitions

Throughout the following description, certain technical terms are usedhaving the following definitions:

"Picture element" (or "pixel"), "image point" and "image segment" allrelate to a delimited zone or area having an optical density capable ofbeing increased and conversely reduced to return to its originalappearance. When the optical density of such an area is increased, thearea will acquire, when viewed by reflection, a coloring or darkening.If the material used to form the area is transparent, the area willbecome partially or completely opaque, when viewed by transmission, uponhaving its optical density increased. When the optical density of suchan area is reduced, the area assumes its original reflectioncharacteristics and/or original transparent quality.

The term "image point" preferably refers to a small area, often ofcircular, square or slightly rectangular shape, repeated on the surfaceof a screen of a display device. Thus, such a display device contains aplurality of image points, each which may represent one node of anetwork which occupies regions or the totality of the area of thescreen. Networks occupying regions of the screen may assume geometricshapes including a square, centered square, and compact hexagon.

The term "image segment" preferably refers to a relatively large displayscreen area. Image segments may be associated, on the surface of adisplay device screen which contains a plurality of image segments, withother image segments of the same shape and/or of different shapeaccording to specific geometric arrangements. One well-known geometricarrangement of image segments is the seven-segment arrangement employedin light-emitting-diode (LED) or liquid crystal display (LCD) devicesenabling representation of numerals 0 to 9 by selective coloring oropacifying of appropriate combinations of these segments.

The term "picture element" (or its usual abbreviation "pixel") meanseither an image point or an image segment.

"Electrochromic modulation" refers to selective illumination anddarkening of a device under electrical control.

"Elementary light-modulating cell" or "elementary light-modulatingdevice" (abbreviated as "elementary device" or "elementary cell") referto the complete structure necessary for electrical control and selectiveillumination of a picture element. An elementary electrochromicmodulation cell comprises the following components, suitably arrangedand associated:

(a) a first transparent electrode, also called the working electrode;

(b) a second electrode or counter-electrode (which may be transparent ornon-transparent, depending on whether the picture element is observed bydirect transmission of light or by reflection);

(c) an ionic conductor or electrolytic material placed between theelectrodes;

(d) means of electro-chromism;

(e) means of electrical connection to an external source of electricalvoltage enabling the cell to be controlled; and

(f) means of addressing (direct, multiplexed, and so forth) enabling itsselective control (present if an elementary cell is part of amultiplicity of elementary cells in one same display device).

In the following text, the expression "elementary modulating cell" ispreferred for denoting the above minimum structure including items (a)through (e) which is required for obtaining a picture element.

The expression "elementary device" denotes a device comprising a singleelementary cell and extrinsic components necessary for its functioning,combined with one or more other elementary devices in a composite devicehaving a plurality of associated cells. Such extrinsic constituents orcomponents include electrical connections associated with theelectrodes; means for masking the periphery of the cell; and atransparent substrate for the transparent electrode.

"Independent light-modulating device" refers to at least one elementarymodulating device and other specific components enabling the device tobe used as an independent unit, including:

(a) mechanical supports or substrates providing the independent devicewith structural rigidity;

(b) a casing;

(c) an encapsulation;

(d) electrical connections inside the independent device;

(e) connection connector(s) or zone(s) to which are connected, by meansof internal electrical connections, elementary devices of theindependent device, enabling easy connection of the latter to thecontrol and addressing electronics and to the associated electricalenergy source;

(f) a printed circuit board possibly able to serve, singly or incombination, as a mechanical support, connector or connector support, orsupport of part of the associated electronics.

Such an independent device can, for example, be a device commonly calleda display device panel or screen, or may be any of variouselectro-optical devices.

"Screen area of a display device" refers to the area which comprises,surrounds and connects all of the picture elements of the device.

"Picture element and elementary modulating cell" also refers to any areawhich can be colored or increased in optical density and thecorresponding elementary cell, whatever their shape and size may be.Such light-modulating devices in some instances do not comprise actualinformation display devices, but instead permit variable transmission orvariable reflection of light. In some such devices only one elementarycell is provided. Such devices include windows, shop windows, screens,windscreens, and spectacles having variable transparency, light valves,shutters, variable reflection mirrors, and light amplification devices.

"Solid consistency" refers to a material having, in the absence ofexternally applied constraints, the appearance of a solid. Solidconsistency includes in a non-restrictive way the consistency of a pastymedium of very high viscosity, the consistency of a thixotropic fluid inthe rest state, the consistency of a gel or a gelled medium and theconsistency of a plastified polymer film. All such materials, whenviewed without disturbance, appear solid.

"Film-forming" refers to forming a sheet of an existing mixture, whichalso includes other constituents.

"Initially water-soluble" refers to a material which is water-solublebefore incorporation in electrolytic material or in a formativecomposition of electrolytic material. Once the electrolytic material isfabricated, the initial water-solubility of the resulting resin can bepartially or totally, reversibly or irreversibly, lost. This loss ofwater-solubility may occur due to cross-linking by a cross-linkingagent, for example.

"Layer" refers to a thin sheet or film of homogeneous, heterogeneous, orcomposite material having a large surface area with respect to itsthickness and preferably having a substantially constant thickness. Sucha layer can be simple or composite; a composite layer itself comprisesseveral layers. Such a layer can be spread undivided, or the layer maybe divided into portions.

"Small thickness" of a layer of electrolytic material refers to athickness preferably between a few microns and a few tens of microns.

"Small thickness" of an electrode refers to a thickness preferablybetween a few hundred Angstroms and several hundred microns.

"Small thickness" of a modulating device refers preferably to between afew microns and a few hundred microns, including the thickness of asubstrate.

"Homogeneous mixture" refers to a mixture whose constituents, on themacroscopic scale, cannot be distinguished from each other, having astructure appearing to be continuous.

"Alloy" refers to the association or combination of several types ofdifferent metals, whether a solid solution, an intermetallic compound, ajuxtaposition of crystallites of each metal, or any other form ofassociation or combination obtained by co-deposition of several metals.

Description of the Preferred Embodiments

As specifically shown in FIGS. 1A and 1B, it was first discovered thatan elementary modulating cell 1 may be constructed by disposing a firstelectrode or working electrode 2 (which electrode 2 is electronicallyconductive, optically transparent) in contact with a face of at leastone layer (or portion of a layer) of at least one electrolytic material3. The material 3 preferably has a thickness between the order of a fewmicrons or a few tens of microns, produced by a homogeneous mixture ofsolid consistency comprising:

a) a water-soluble salt or a water-soluble mixture of salts of at leastone metal which can be cathodically deposited from an aqueous solutionof one of its simple or complex ions;

b) at least one initially water-soluble film-forming polymer resin,preferably in the proportion of one part by weight to 0.05 to 50 partsof anhydrous salts; and

c) water.

Such a cell further includes a second electrode or counter-electrode 4,in contact with the other face of the layer 3 electrolytic material.

Illuminating a picture element is accomplished by "writing" the element.The "writing" process comprises increasing the optical density of theinterface region between the working electrode 2 and the layer ofelectrolytic material 3. It is possible to write a picture element 5 byapplying to the working electrode 2 a negative electrical voltage ofbetween a fraction of a volt to a few volts with respect to thecounter-electrode 4 for a time interval having a certain duration. Thistime interval is referred to as the write phase, and the state of a cellduring the write phase is illustrated in FIGS. 2A and 2B.

Further, it is possible to maintain an image point or image segment aswritten during a certain time while removing the applied electricalvoltage. Such maintenance of optical density while eliminating theexternally applied potential difference is referred to as themaintaining phase and is illustrated in FIGS. 3A and 3B.

It is also possible to maintain the image point or image segment in thewritten state for a duration longer than the maintenance statepreviously described, by applying a write voltage which is close to theelectromotive force which the elementary cell may exhibit in the writtenstate.

It is also possible to erase, either partially or totally, the increasedoptical density of a written picture element 5. Such erasure occursduring an "erase phase" and comprises reducing or eliminating theincrease in optical density obtained during the previous write phase, bycausing an electric current to flow between the electrodes 2, 4 in adirection opposite the direction of current flow applied in the writephase, as illustrated in FIGS. 4A and 4B.

Additionally, it is possible to maintain the picture element 5 in theerased state.

Finally, it is possible to repetitively restart the write, erase andmaintaining phases.

In addition to the foregoing characteristics, the electrolytic materialused in the elementary cell has the following further characteristics:ionic conductivity; plastic or viscoelastic deformability; and it can beconformed in a continuous layer of small thickness. Further, theelectrolytic material not only participates in the writing or erasing ofan image point or image segment. It can also enable the flow ofelectrical current between the working electrode and thecounter-electrode during the writing and during the erasure,accomplishing this in the following ways:

(a) by the cathodic reduction of depositable metallic ions which arecontained in the material, in combination with the working electrode(write phase);

(b) by anodic oxidation and re-incorporation into the original state ofthe metallic ions produced by this oxidation in combination with theworking electrode (erase phase); and

(c) by carrying in each direction electrical charges in ionic formbetween the working electrode and the counterelectrode in quantitiesequal to the electrical charges injected in electron form into theelectrolytic material by the working electrode during writing, and tothe electrical charges extracted in electron form from the electrolyticmaterial by the working electrode during erasure.

An electrochromic light-modulating process implemented by the cell andthe material thus described comprises the following steps:

(a) producing at least one such electrolytic material;

(b) producing at least one layer of at least one electrolytic materialhaving a thickness of between a few microns and a few tens of microns;

(c) disposing the electrodes of the cell in contact with the faces ofthe layer of electrolytic material;

(d) during at least one write phase of a certain duration, applying anelectrical potential negative with respect to that of thecounter-electrode to the working electrode, thereby causing at least oneimage point or one image segment to be written; and

(e) during at least one erase phase, after the write phase, passing anelectrical current having a polarity opposite to that of the electricalcurrent during the write phase, such that the picture element previouslywritten is erased.

The process is repetitive and is able to comprise several pairs of writeand erase steps.

Surprisingly, the written picture element 5 is on the one hand an areadelimited by a remarkably sharp contour, corresponding to theintersection of the orthogonal projections, on the screen area of thecell, the areas corresponding to the areas of the two electrodes 2, 4and of the layer of electrolytic material 3 between them. On the otherhand, the picture element has a dark, matt and amorphous appearance,very different from that of a conventional metallic deposit. It appearsclear, bright and crystalline, close to that of a metallic "black?, andhaving the appearance of black printing ink, as described below.

For example, if the counter-electrode 4 has an area whose projection isincluded inside that of the working electrode 2 and of the layer ofelectrolytic material 3, the written picture element 5 exactlyreproduces the shape of the counter-electrode 4, even though the writtenpicture is formed at a certain distance from the counter-electrode. Onedoes not observe any blurred or diffused contour, as could be expected.

Furthermore, the picture element 5, once written, does not diffusebeyond its contour and is not diluted by its undarkened surroundings.This characteristic is illustrated diagrammatically in FIG. 5. Theobtaining, under such conditions, of a sharp contour of the writtenimage point or image segment 5 is very important. Although it is in factpossible with known electrochromic processes to obtain picture elementswith a sharp contour by delimiting the transparent electrode or theelectrochromic material, in common cases in which the electrochromicmaterial is a thin solid deposited layer on the transparent electrode,this delimitation, makes the manufacturing complex and expensive andreduces the resolution and the average contrast. This is true,particularly in the case of a so-called matrix display device whichcomprises a matrix of pixels placed at the intersections of a system ofhorizontal conductive rows and vertical conductive columns.

A delimitation of the contour of each picture element 5 which can bereduced to the intersection of the areas of a transparent electrode 2and of a counter-electrode 4, both elongated and having directionsgenerally perpendicular to each other, the electrolytic material 3having an area covering at least this intersection. This delimitationlends itself to extremely simple construction and enables obtaining ofpicture elements 5 very small in size, as well as minimizing gapsbetween adjacent picture elements. This variant is representeddiagrammatically in FIG. 9 in the particular case in which theelectrolytic material 3 is in the form of strips which are coaxial withthe counter-electrodes 4 and slightly and laterally overlapping thelatter. This variant is intended for so called matrix display panels.

Surprisingly, the optical density of the picture element 5 is uniforminside its contour up to large sizes of this picture element, on theorder of several square centimeters, without special precautions. Beyondthis size, because of the resistivity of the working electrode 2, anappropriate geometry of the electrical current supply zones or points(that is, electrical current lead zones and 12, respectively, on theworking electrode 2 and possibly on the counter-electrode 4) isnecessary to ensure a sufficiently uniform current density in order toobtain a uniform optical density.

It is possible to obtain a coloring or densification of the pictureelement exhibiting by reflection a very high optical density (a printingink or Indian ink black appearance), and a total opacity bytransmission. But, surprisingly, it is also possible, by causing to flowan electrical charge which is smaller than that enabling the maximumoptical density or complete opacity to be obtained, to obtain a loweruniform optical density. This lower uniform optical density is a greycoloring when viewed by reflection, and grey screen or filter uniformlyreducing the transmission of light when viewed by transparency. Moreprecisely, it is possible in both modes of vision (transmission andreflection) to produce, by varying the electrical write charge, acontinuous grey scale going from the initial absence of coloring oroptical density when viewed by reflection, or from initial transparencywhen viewed by transmission, respectively, to a high optical density, inparticular a dense black or opacity. Obtaining of such a grey scale isimportant, particularly for the display of high quality graphicalimages.

In contrast, according to document FR 2,504,290, a mark obtained with awriting stylus is always very dense by reflection and opaque bytransparency. The fact of varying the electrical charge when the writingstylus forming a cathode is immobile during the writing of a point, orof varying the current density when the stylus is moving for writing aline, has the sole effect of varying the diameter of the point or thewidth of the line. However, it does not affect its optical density as itdoes according to the present invention. It is possible to obtain,starting from the erased state, an increase in optical density with aduration of application of the electrical write phase voltage of only afew milliseconds, and a corresponding decrease with an erase phaseduration of the same order of magnitude.

The writing process can be implemented in such a way as to exhibit awell-defined electrical voltage threshold of high value. That is, whenapplying an electrical write voltage less than this threshold to anelementary cell 1, the picture element 5 is not written. Such anelectrical writing voltage threshold is essential for the multiplexedaddressing of a 15 matrix display device.

The erasure of the written picture element 5 is obtained by causing toflow in the elementary cell 1 a current of opposite direction to that ofthe write phase current. The erase phase current is generally obtainedeither by applying to the electrodes 2, 4 an electrical voltage of theopposite direction to that of the writing voltage; or, in the case inwhich the cell exhibits an electromotive force, by simple shortcircuiting.

The present modulation process enables, in a reversible and repetitiveway, writing and erasing of image points or image segments, which is notforeseen in the document FR 2,504,290, referred to above.

A picture element 5 is written by cathodic reduction in the region ofthe interface between the working electrode 2 and the layer ofelectrolytic material 3. The cathodic reduction involves metallic ionspresent in the layer of electrolytic material 3 being reduced into ametal or metallic 10 alloy which electrocrystallizes according to aparticular mode having remarkable characteristics of optical density,uniformity, sharpness of contour, grey scale and absence of diffusion ordilution, and so forth, as described above.

Picture element 5 is erased by anodic oxidation of the metal or metallicalloy plate formed as described above. The electrolytic material is thusalready characterized by enabling both the writing and erasurefunctions.

Surprisingly, it is observed that the metallic deposit formed in theinterface region is capable of being redissolved by anodic oxidation,without a residual deposit remaining. The redissolution is also achievedwithout massive reinjection of metallic ions produced by this oxidationinto the electrolytic material, thereby avoiding production of harmfulor parasitic phenomena or processes (such as a flocculation or a local"salting out" of the film-forming polymer), as would have been expected.

The cathodic deposit of metal or alloy according to a particular mode ofelectrocrystallization observed, and its dissolution by anodicoxidation, is obtained with layers of electrolytic material containing(depending on the cases) a single metal or several metals chosen frommost of the metals which can be cathodically deposited alone orco-deposited with several or which cannot be deposited alone but can becojointly deposited with others, from an aqueous solution of theirsimple or complex ions or a combination of them. In particular, thefollowing metals are suitable: zinc, cadmium, lead, silver, copper,iron, cobalt, nickel, tin, indium, platinum, palladium, gold, bismuth,antimony, tellurium, manganese, thallium, selenium, gallium, arsenic,mercury, chromium, tungsten, molybdenum, associated with a large numberof water soluble film forming polymer resins. The actual crystallinestructure of the developed metallic deposits, which appears interspersedin the network of the polymer resin, could be that of a highly dividedstate with regard to appearance and optical density. One of thehypotheses is that of a multi-dendritic growth along the molecularchains of the resin. However, the invention is not tied to thehypotheses and assumptions thus mentioned.

The present electrolytic material can contain (without disadvantage forobtaining of the optical densification of the picture element with thecharacteristics mentioned above and without disadvantage for itserasure), in addition to the already mentioned electro-depositablecations, cations of metals which cannot be electro-deposited from anaqueous solution in substantial proportions. This characteristicproduces a greater flexibility in the formulation of electrolyticmaterials better responding to various individual applicationrequirements.

In contrast, according to document FR 2,504,290, writing marks on therecording medium is inhibited and replaced by a metallic plate on thecathode comprising the writing electrode. And/or it is inhibited also bya release of hydrogen, when the layer of electrochromic materialcontains a considerable proportion of metal cations which cannot beelectro-deposited from an aqueous solution, such as the alkali metals(with the exception of ammonium ion), the alkaline-earths in particular.

In the present material, the presence of a considerable quantity ofmetal cations which cannot be electro-deposited has no inhibitingeffect. This could be associated in particular with the fact that thevoltages necessary for writing are a maximum of a few volts (whileaccording to the document FR 2,504,290, the voltages used are generallywithin a range from about a few volts to a few tens of volts).

A layer of the present electrolytic material is generally a continuouslayer. That is, it is generally non-granular, and is transparent orsubstantially transparent. Depending on the nature of the ions which itcontains, it can be colorless or colored.

In the case in which the elementary cell operates by transmission, inwhich case the counter-electrode is also a transparent electrode, thelayer of electrolytic material is left in this transparent form or in aform which is substantially transparent or as slightly colored aspossible. This is true unless it also constitutes a colored filter, forexample, for producing variable transmission colored apertures or colordisplay devices.

In the case in which the elementary cell operates solely by reflection,it is generally necessary to add to the layer of electrolytic material amasking and/or contrasting pigment compatible with the otherconstituents of the electrolytic material. Such a pigment has the effectof masking the counter-electrode if the latter does not constitute abackground of satisfactory color and/or contrast, while attenuating apossible colored tint of the electrolytic material if such a coloringexists because of the composition and is not desirable, and ofconstituting a background providing the most desirable contrast with theblack appearance of the written picture element. A white pigment such astitanium dioxide, particularly in the rutile and anatase crystallineforms, dispersed in particulate form in the layer of electrolyticmaterial or in only a section of the thickness of this layer, enablesattainment of a particularly white background. For the purpose ofobtaining a particular colored background (which can be of particularinterest for production of color display devices) it is possible to usecolored pigments alone or mixed with a white pigment.

Surprisingly, the color of a colored pigment or of a dye present in theelectrolytic material is progressively extinguished until it ispractically black, without residual coloration, when the optical densityof a picture element is progressively increased. Everything happens asif the reflected or transmitted colored light (depending on the case)were filtered by the neutral grey screen produced by the written imagepoint or image segment. This remarkable feature enables the productionof multi-color display devices by three-color additive synthesis usingthree electrolytic materials, each material colored according to one ofthe three primary colors.

The present electrolytic material can comprise, depending on the cases,the ions of a single metal or of several metals chosen among most of themetals which can be cathodically deposited alone or co-deposited withseveral. It is therefore possible, depending on the case, to change theconditions for obtaining the deposit of a single one of these metalsand/or modify the write or erase features and/or modify the appearanceof such a deposit, or obtain by cathodic reduction an alloy which canhave an appearance and/or write or erase characteristics and/or featurescombining those of the individually deposited metals, but also have anappearance and/or characteristics and/or write and erase features whichare completely new. This may be true, for example, with regard to"memory" (that is, nonvolatility of memory, the persistence of thecoloring, densification or opacification of the picture element in theabsence of electrical voltage applied to the elementary cell).

According to a preferred embodiment of the electrolytic material, thewater-soluble metallic salt or water-soluble mixture of metallic saltsis hygroscopic and preferably deliquescent in the presence ofatmospheric humidity. According to this preferred embodiment, a layer orfilm of electrolytic material having a thickness of between a fewmicrons and a few tens of microns not enclosed in a sealed enclosurepermanently retains (down to a very low atmospheric humidity) a highionic electrical conductivity which enables the elementary modulatingcell to be operated with a minimum voltage of a few volts. This highionic electrical conductivity is due to the fact that, with hygroscopicsalts, the layer of electrolytic material, although having theappearance and solid consistency of a dry layer, retains a certainquantity of water in equilibrium with atmospheric humidity. Thisinternal water, in which the metallic salts are dissolved in very highconcentration, provides the layer of electrolytic material withconsiderable ionic conductivity. The ionic conductivity varies withatmospheric humidity, but remains high down to its very lower levels ofwater content. It retains a conductivity value which depends on thedegree of hygroscopicity or deliquescence of the chosen combination ofsalts.

According to this preferred embodiment of the composition of theelectrolytic material, it is possible to avoid sealing thelight-modulating elementary cell in a strictly leak-tight way, unlikemost known electrochromic display devices. This avoidance of leak-tightsealing represents a considerable simplification in manufacture of thecell or of the device, and provides a reduction in cost.

In fact, it is possible to tolerate the effects of a penetration ofatmospheric humidity into the cell and the effects of a loss of watercontained in the layer of electrolytic material to a very large degree.Surprisingly, it is also possible to tolerate the effect of apenetration of atmospheric oxygen into the cell. It could have beenfeared that, during the operation of an elementary light-modulating cellwhich is not sealed strictly leak-tight, variations in relativeatmospheric humidity which can give rise to variations in the watercontent of the electrolytic material, could produce significantvariations of impedance of the elementary cells, thus giving rise tocorresponding fluctuations in the electro-optical characteristics.

But, in fact, everything happens as if, with the thicknesses consideredfor the layer of electrolytic material, the impedance variations due tovariations in the resistivity of the electrolytic material were, over awide range of variation of the latter, secondary with respect to theoverall impedance of the cell. The impedance, like other factorscontributing to the impedance of the cell, comprises multiplepolarizations corresponding to various electrochemical processesoccurring at each electrode (in particular electrochemical activationpolarizations, concentration polarizations). In any case, it is possibleto compensate for a substantial variation in the overall impedance ofthe cell by modifying the electrical writing voltage.

A protective insulation of the cell o of the device is desirable or evennecessary when the light-modulating cell is operated in extreme and/oraggressive and/or corrosive atmospheric environments, for the purpose oflimiting or preventing components of the cell or device from coming intocontact with the external medium. But the effect on the structure,manufacture and cost is very different from that of 10 having to provideeach cell, group of cells or device with strictly leak-tight sealing orprotection which is capable of providing and retaining a strictlyleak-tight protective insulation despite the thermal or mechanicalstresses to which the cell or device may be submitted.

A layer of the present electrolytic material which has a solidconsistency in the absence of externally applied stresses, has, underthe effect of such stresses, a plastic or viscoelastic behavior. Theelectrolytic material's characteristics depend particularly on thenature of the polymer resin and the degree of cross-linking.

This plastic or viscoelastic behavior is very important. On the onehand, it enables the layer of electrolytic material to be shaped tocompensate for defects in the flatness of one or both of the electrodes,and to compensate for defects in parallelism between the two electrodes.This behavior thus provides an excellent physical and electricalcontact, despite these defects.

On the other hand, at the interface between the working electrode orcounter-electrode and the layer of electrolytic material, theelectrolytic material remains connected. A good physical and electricalcontact remains assured even if overall or local strain affects the cellor light-modulating device, due to compliance of the electrolyticmaterial.

Furthermore, this plastic or viscoelastic behavior increases "lifetime";that is, the number of accessible write-erase cycles. It is known thatthe write and erase reactions of an electrochromic device produce localstresses and strains due to morphological changes associated with theseelectrochemical reactions. At the interface of two solids which can bothpresent only elastic distortions under the considered conditions(particularly fragile solids), even small strains can give rise to highstresses whose cyclic repetition is capable of altering the quality ofthe contact (especially of the electrical contact). It is also capableof reducing the lifetime of the device.

This disadvantage of known electrochromic devices where theelectrochromic material (and in certain cases the electrolyte) arefragile solids, does not affect devices made in accordance with thepresent invention. The quality of the contact is maintained at theinterface of each working electrode (or counter-electrode) and the layerof electrolytic material. Quality of contact is maintained because ofthe compliance of at least one of these solids.

The above-mentioned physical properties of the electrolytic materialenable a considerable simplification of the construction of alight-modulating cell or device, and a reduction in requirementsrelating to the constitutive materials and components. The propertiesalso enable production of very large display panels. In fact, it is notnecessary to provide a specific spacer intended to maintain a strictparallelism and an accurate spacing between the working electrode andthe counter-electrode. A layer of electrolytic material deposited byindustrial application or known coating techniques generally (such asair gap, coating bar, scraper, exclusion, calendaring and silkscreening, for example) suffices to constitute the spacer and to definea sufficiently accurate spacing.

The substrate of the transparent working electrodes can be, withoutdisadvantage, for example, a plate of drawn glass. Very large-areadisplay panels (comprising a multiplicity of elementary display cells)can be constructed; yet the strains (mechanical, thermal, vibratory) towhich large areas are likely to be subjected do not have a harmfuleffect on the physical integrity and functioning of these elementarydisplay cells

By an appropriate choice of the film-forming water-soluble polymerresin, and taking into account other composition factors, the presentelectrolytic material has adhesive properties. More specifically, theelectrolytic material has a sticky touch (known by the expression"tack") or contact adhesion (known by the expression "pressure-sensitiveadhesion"). Such resins can be (in particular nonlimiting examples)hydroxyethylcellulose, polyvinylpyrrol idone, polyvinyl alcohol, orequivalents.

In the presence of a high concentration of a contrasting pigment, or inthe case of the use of a water-soluble polymer resin which does notproduce this tack, or when the polymer resin is strongly cross-linked inthe electrolytic material, the surface adhesion can be greatly reducedor non-existent. In these cases, the layer of electrolytic materialadvantageously comprises a composite with three superimposed layers. Twoexternal layers of the three are formulated by means of an appropriateresin, and contain neither a crosslinking agent nor contrasting pigment,or sufficiently little not to affect the tack; the internal layer 7 isable to be deprived of this tack (FIG. 6).

The existence of such a tack or surface adhesion also enablessimplification of the manufacture of light-modulating cells and devices.In fact, the mechanical cohesion of each cell can be maintained solelyby the pressure-sensitive adhesion properties of the layer ofelectrolytic material; the material adheres both to the workingelectrode and to the counter-electrode so that it is unnecessary toprovide additional external mechanical means to support the cell.

Furthermore, the adhesion of the electrolytic material to the twoelectrodes (working electrode and counter-electrode) provides anexcellent physical and electrical contact of the electronic conductorand the ionic conductor at each interface. It is thus unnecessary toapply and maintain a pressure on the cell, or to provide mechanicalmeans for this purpose.

The combination of such a pressure-sensitive adhesion and the plastic orviscoelastic deformability already mentioned enables the production oflarge-sized display panels which can have a very simple structure andwhich are not affected by thermal and mechanical strains and vibrationsto which such panels can be subjected.

In the functioning of an elementary electrochromic cell, there occurs atthe counter-electrode an electrochemical reaction corresponding to thatwhich occurs at the working electrode. One electrochemical reaction isan anodic oxidation if the other is a cathodic reduction, andvice-versa. There must therefore exist at the counter-electrode anauxiliary redox couple capable of reversibly changing from one of itsterms to the other by electrochemical oxido-reduction. In the absence ofsuch a reversible auxiliary redox couple, the oxidation and reduction atthe counter-electrode can lead to degradation of the constituentmaterials of the cell and/or to generation of gaseous species whichinterfere with operation.

The present electrolytic material already intrinsically contains atleast a first auxiliary redox couple which is precisely the redox couplewhich is implemented at the working electrode: the metallic ion(s)-metalor alloy couple. However, prior to operation of the cell, the same termof the couple is present at the working electrode and at thecounter-electrode, while operation requires the presence of conjugatedterms. It suffices, for example, to initially apply a sufficient voltagefor a few seconds in order to create the necessary asymmetry for thecell to function correctly: everything functions as if the cell alwayscontained a sufficient quantity of electroactive oxidizable species toenable such an asymmetry without damage.

An auxiliary redox couple of this type enables a satisfactoryfunctioning by reflection if there is incorporated in the electrolyticmaterial a masking pigment which conceals the counter-electrode. Infunctioning by transmission (with an electrolytic material and acounter-electrode which are kept transparent), erasure of the visibledeposit on one of the electrodes is accompanied by the formation of avisible deposit on the other electrode, and the maximum transmission ofthe cell is reduced.

The present electrolytic material can also contain, intrinsically, asecond auxiliary redox couple whose reduced form is on the one handwater-soluble in the presence of other water-soluble constituents of thematerial and, on the other hand, colorless or only slightly colored atthe concentrations used. An auxiliary redox pair of this type enables asatisfactory functioning both in transmission and in reflection,avoiding in transmission the disadvantage mentioned above.

The intrinsic presence of this second auxiliary redox pair in theelectrolytic material can have two origins. A first origin is when asimple or complex metallic ion which is cathodically reducible to metal(introduced as such into the electrolytic material) can also reversiblychange to a higher degree of oxidation. This is the case, among others,of lead, silver, copper, iron, mercury and tin in particular. Forexample, Cu(I), Fe(II), introduced to respectively create theelectrochromic processes

    Cu(I)←→Cu(II), Fe(II)←→Fe(O)

create at the same time the auxiliary redox pairs

    Cu(I)←→Cu(II) and Fe(II)←→Fe(III).

A second origin or the intrinsic presence of a second auxiliary redoxpair in the electrolytic material is when one of the anions of thewater-soluble mixture of salts of the electrolytic material canreversibly switch to a higher degree of oxidation. This is particularlythe case of the halide anions. For example, the presence of the chlorideor bromide anion creates same time as the auxiliary redox couples suchas

    2Cl.sup.- ←→Cl.sub.2, 3Br.sup.- θ→Br.sub.3.sup.-.

It is also possible to introduce extrinsically into the presentelectrolytic material (if it does not contain it intrinsically) anauxiliary redox couple of the previous type. That is, an auxiliary redoxcouple whose reduced form is water-soluble in the presence of otherwater-soluble constituents of the material, and colorless or onlyslightly colored in the concentrations used.

Presence of such an auxiliary redox couple in the electrolytic materialcorresponds to a preferred composition of the electrolytic materialwhich is particularly advantageous. In fact, it is found that itsuffices for the counter-electrode to have simple electronic conductionproperties (and optical transparency properties if the cell is intendedto function by transmission), unlike the counter-electrodes of complexcomposition and structure of many known electrochromic devices. Numerousmaterials, particularly commercially available materials, can thus bedirectly suitable as counter-electrode materials.

Finally, the electrolytic material can be associated with acounter-electrode itself having redox properties. For example, acounter-electrode may be formed from a metal which can be anodicallyoxidizable in a reversible way. One example is lead corresponding withthe redox pair:

    Pb←→PbO.sub.2

Or, a counter-electrode may be formed, covered with a layer of an oxideor solid compound capable of reversibly changing between two differentdegrees of oxidation.

The present electrolytic material may be a continuous material (that is,not granular), is transparent, can be made contrasting and opaque in itsmass, is of solid consistency, has plastic or viscoelasticdeformability, has permanent ionic conductivity which it is furthermorecapable of retaining even if it is exposed to the atmosphere, and isalso capable of exhibiting a pressure-sensitive adhesion. The material,when conformed in a layer or thin film, preferably has a thickness offrom a few microns to a few tens of microns. The material is placed incontact with a first (or working) transparent electrode on one side, andwith a second electrode (or counter-electrode) on the opposite side.

The material constitutes and comprises the following: the electrochromicmaterial, the electrolyte, and an auxiliary redox couple of thelight-modulating cell thus constituted, possibly a spacer, a means ofcohesion for the cell, and a means for maintaining internal electricalcontacts.

As an electrochromic material, it is capable of undergoing (at theinterface with the transparent working electrodes) a reversible changeof the degree of oxidation accompanied by a reversible change ofcoloring and/or optical density. By cathodic reduction, a metal or ametal alloy is formed at the interface, thus appearing as a darkening oropacification constituting a picture element having a remarkable set ofcharacteristics. By anodic oxidation, the metal or alloy is redissolvedinto metallic ions, thus restoring the initial appearance of the medium.

As an electrolyte, it has a high ionic conductivity due to its veryconcentrated aqueous solution nature, a conductivity which it retainspermanently even without strictly leak-tight sealing in a preferredembodiment.

As an auxiliary redox couple, it allows an electrochemical reaction totake place reversibly at the counter-electrode. This reaction is theconjugate of that which occurs at the same time at the workingelectrode.

Such an electrolytic material comprises a homogeneous mixture of solidconsistency comprising:

a) at least a water-soluble salt (or a water-soluble mixture of salts)of at least one metal which can be cathodically deposited from anaqueous solution of one of its simple or complex ions;

b) at least one initially water-soluble film-forming polymer resin,preferably in the proportion of one part by weight to 0.05 to 50 partsof anhydrous salts; and

c) water.

The electrolytic material may additionally comprise, as necessary, thefollowing items (to which the inventive material is not to be limited):at least one additional redox couple; cations which cannot beelectrodeposited in aqueous solution; at least one solid in dispersedparticulate form (in particular a contrasting and/or masking pigment);at least one coloring agent; at least one acid; at least onecross-linking agent; at least one complexing agent; at least onedissolved or dispersed additive capable of improving the properties anduse of the electrolytic material; and at least one agent for theformation and/or application as a layer or film of the electrolyticmaterial.

According to a variant embodiment, the layers (or films) of electrolyticmaterial can be divided into at least two superimposed or interleavedlayers, each containing a different percentage of each constituent.

According to another variant embodiment, it is possible to fabricate alayer or film of composite material by superimposition or inter-leavingof at least two different electrolytic materials. For example, it can beadvantageous to fabricate a composite layer comprising a non-adhesivefilm but mechanically very solid, comprising, for example, a resin suchas sodium carboxymethylcellulose cross-linked in the film, and one ortwo external layers which are less mechanically solid but havepressure-sensitive adhesion, comprising, for example, polyvinylpolyvinylpyrrolidone or hydroxyethylcellulose.

According to another variant, the material comprises at least twoelectrolytic materials, each as previously defined.

The materials which can be used have been described above. It ispossible to use, depending on the case, a single metal or several metalschosen from the metals which can be cathodically deposited from anaqueous solution, and metals which individually do not deposit ordeposit poorly, but which co-deposit with certain of the previousmetals: tungsten and molybdenum in particular.

The metallic salts which can be used are ionic compounds in which themetal is present in cationic form or incorporated in a cationic complex;the anions of these compounds, and other conditions (particularly thepH), are chosen such that the compounds are substantially completelysoluble in an aqueous medium.

Appropriate anions could be found, for example, among the following:chloride, nitrate, sulphate, borate, fluoride, iodide, bromide,fluoroborate, fluorosilicate, fluorogallate, dihydrogenophosphate,chlorate, perchlorate, bromate, selenate, thiosulfate, thiocyanate,formiate, acetate, butyrate, hexanoate, adipate, citrate, lactate,oleate, oxalate, propionate, salicylate, glycinate, glycocollate,glycerophosphate, tartrate, acetyl-acetonate, isopropylate, benzoate,malate, benzene sulphonate, 1-phenol-4-sulphonate, in particular.

The salts which can be used can also be ionic compounds in which themetal forms an outer orbital complex anion associated with a cation(such as, for example, the ammonium ion). Examples of such anioniccomplexes are the chloropalladate ion, the chloraurate ion, and thestannate ion, in particular.

Regarding the preparation of the materials containing a salt or amixture of hygroscopic and preferably deliquescent salts, most of themetals have some of their salts which are hygroscopic or deliquescent,most of them halides, nitrates, perchlorates, chlorates andthiocyanates, in particular.

A deliquescent mixture of salts is generally obtained from individuallydeliquescent salts, but mixtures can be deliquescent without theirconstituents being deliquescent themselves. The mixture can be moredeliquescent (that is, crystallize at a lower relative humidity) thanthe most deliquescent of the constituents.

The presence of halide anions (chloride, bromide, iodide, fluoride) inthe electrolytic material is found to be particularly advantageous, whensuch a presence is compatible with the other constituents of thematerial and the expected properties. It is found, in fact, that thepresence of halide anions is often advantageous with particular regardto deliquescence, auxiliary redox function, solubility of metallic ions,ease of electrodeposition and re-dissolution of the metal or metalalloy, and reversibility of the write-erase process.

Use in the material of an association of salts of various depositablemetals offers wider possibilities than the use of a single metal.

First, certain metallic salts which cannot be used (or which aredifficult to use alone) in relatively high concentration become easy touse in a low-concentration mixture with others, where their specificlimitations (for example, solubility, coloring, and so forth) becomeacceptable or inapparent.

It is also possible to change, in certain cases, the conditions in whichan electro-depositable cation is deposited, and/or to modify the writeand erase features and/or the appearance by the presence of othercations.

Finally, by association of several different metallic ions, it ispossible to obtain, by cathodic reduction, deposition of at least twometals as an alloy whose properties, appearance, optical density andreflection can be completely different from those of the individualmetals. It is thus possible to obtain new and multiple write and erasefeatures. In particular, new features include those regarding memory(that is, "nonvolatility," or persistence of the written picture elementafter removal of the writing voltage), existence and value of a voltagethreshold (that is, a minimum writing voltage), and more generally ofvarious non-linear characteristics which are particularly advantageousfor matrix addressing without loss of contrast nor cross-talk of displaydevices comprising a large number of picture elements.

In the electrolytic material, it is found that the presence of copperions (even in very low relative concentration) in association with othermetallic ions is particularly advantageous with particular regard toease of electrodeposition and re-dissolution, reversibility of thewrite-erase process, and the appearance of the deposit.

It has been unexpectedly found that a high ratio of water-soluble(anhydrous) salts to the water contained in the electrolytic material(preferably higher than 0.05, and more preferably even higher than 1)had a favorable effect on various operating characteristics of the cell.Particularly, favorable effects were observed with respect to appearanceand optical density of the written picture elements, polarizations andreversibility.

Usable initially water-soluble film-forming polymer resins compriseresins capable of forming actual aqueous solutions. Also, resins capableof forming a colloidal dispersion in water are usable. It is possible toquote (by way of non-limitative and purely indicative examples) polymerssuch as polyoxyethylene, polyvinylpyrrolidone, polyvinyl alcohol, thecellulosic ethers such as, for example, hydroxyethylcellulose andcarboxymethyl cellulose, sodium alginate, polyacrylic acid and itsderivatives, gelatin, gum arabic, polystyrene sulfonic acid,polyacrylamide, in particular several resins which are compatible witheach other (that is, which are not co-precipitant) which can be used ina mixture.

Preferably, the molecular weight of the resins is between 10,000 and10,000,000. The mechanical qualities of the layer of electrolyticmaterial can be improved, as far as desired, with a resin having amolecular weight located in the upper section of the indicated range(towards 10,000,000). The polymer resin, in addition to its functions inthe layer of electrolytic material, provides the material or fluidformative composition with a viscosity which facilitates application inthin layers, a viscosity which can be adjusted in various ways. Inparticular, it is possible to use polymer resins which provide theelectrolytic material with pressure-sensitive adhesion properties (tackor contact adhesion), such as (for example, in a non-limitative andpurely indicative way) hydroxyethylcellulose, polyvinyl alcohol andpolyvinylpyrrolidone, either for constituting a single layer or film, orfor constituting at least one external layer of a composite film.

The quantity of water is such that, on the one hand, the electrolyticmaterial retains its solid consistency in the absence of externalstresses, and on the other hand its ratio to the water-soluble salts isas has been mentioned above.

The cations of metals which are not electro-depositable from an aqueoussolution can be chosen (in a non-limiting and purely indicative way)from the alkali metals, the alkaline-earths, aluminum, beryllium, mostof the rare earths and, in general, the cations of highly reducingmetals which cannot be electro-deposited in aqueous solution. They alsocomprise the cations non-reducible into a metal such as, for example,the ammonium ion, the quaternary ammonium ions in particular.

The water soluble salts of these cations must be understood aswater-soluble in the presence of other water-soluble salts of thematerial. That is, they may be chosen such that the mixture of all ofthe salts present in the material is water-soluble.

It was unexpectedly found that one or more solids could be homogeneouslydispersed in the electrolytic material in particulate form for, inparticular, improving or modifying mechanical properties, appearance ofthe written picture element, diffusion, and reflection of light. Inparticular, such a solid is a masking and/or a contrasting pigmenthaving certain functions. The functions include masking thecounter-electrode if the counter-electrode does not constitute abackground of satisfactory color and/or contrast, functioning as abackground providing the most desirable contrast with the blackappearance of the written picture element (for example, masking theblack appearance of a counter-electrode containing carbon andsubstituting for it a contrasting background, most often white),attenuating a possible colored parasitic tint of the electrolyticmaterial by swamping it. Further functions include creating a particularcolored contrasting background for producing colored light-modulatingdevices, especially for creating three-colored backgrounds, eachaccording to the three primary colors of an additive three-colorsynthesis process for producing multi-color display devices.

Numerous inorganic and organic white and colored pigments can be used,provided that they do not interact chemically with the otherconstituents of the electrolytic material. Titanium dioxide, principallyin the rutile and anatase crystalline forms, is a white pigment which isremarkably stable in most of the present electrolytic media, as it has avery high hiding power and a high whiteness index. This titanium dioxidepigment can be used in conjunction with a colored pigment, in a mixture,or by superimposition in a composite layer of electrolytic material, soas to enable modification of the saturation of the color and/oradvantage to be taken of its high hiding power for a colored background.Such colored pigments are, for example, zinc chromate, minium, cobaltblue, and chromium oxide. It is similarly possible to associate titaniumdioxide and a soluble dye. According to the nature of the pigment, itsparticle size, color, hiding power, and the desired effects, the ratioof pigment dispersed in the electrolytic material can vary between widelimits, preferably between 0.1 and 50 parts by weight of pigment for onepart of film-forming polymer resin.

If necessary or desired, in the case of a light-modulating cellfunctioning by way of transmission, the electrolytic material cancomprise one or more dissolved or dispersed dyes, for the purpose ofproducing colored transmission filters of variable transparency. Thiscomposition may be used for modulating devices such as coloredtransmission apertures, variable transparency apertures, and/or coloreddisplay devices. More particularly, it may be used for producing threefilters, each colored according to one of the primary colors of theadditive three-color synthesis system, for multi-color display devices.Numerous colorants, particularly of the type used for gouaches and watercolors, can be used, provided they have no chemical interaction with theother constituents of the electrolytic material.

As mentioned above, the electrolytic material can comprise, ifnecessary, one or two additional auxiliary redox couples whose reducedform is on the one hand water-soluble in the presence of the otherwater-soluble constituents of the material and, on the other hand,colorless or slightly colored at the concentrations used; theelectrolytic material has the functions of modifying the writing voltagethreshold, improving reversibility of the write-erase process, andincreasing the number of write-erase cycles. It is possible, forexample, to use metallic species having two degrees of oxidation, whosereduced form is soluble (or can be made soluble, for example, bycomplexing in an aqueous medium).

The electrolytic material can further comprise an acid in sufficientquantity for maintaining the pH at a suitable value, and preventinghydrolysis and/or precipitation of the metallic species present and/orgelling or syneresis or flocculation of the resin in the electrolyticmaterial. For example, it is possible to use (in a non-limitative andpurely indicative way) hydrochloric acid, hydrobromic acid, acetic acid,perchloric acid, chloric acid, formic acid, nitric acid, or anequivalent.

The electrolytic material can further comprise a crosslinking agent forthe polymer resin. Inclusion of a crosslinking agent may be used forstrengthening the electrolytic material's mechanical qualities,particularly hardness and cohesion. For this purpose, it is possible touse the usual cross-linking agents for polymer resins, such aspolyfunctional resins and compounds (for example, glyoxal,dimethylolurea, epoxy compounds, carbodiimide, isoxazole and dialdehydestarch in particular).

It is also possible in the case of resins containing, for example,carboxylic groups (like sodium carboxymethylcellulose), to usedepositable polyvalent cations like Zr(IV), Sn(IV), and Al(III), inparticular. In the case of Al(III), for example, the cation weight canpreferably vary between 0.01 and 0.5 parts per one part of resin. Withnumerous polyvalent metallic ions usable according to the invention,cross-linking is obtained spontaneously without it being necessary toadd an additional reticulating agent. The conditions for using thecross-linking agent are chosen in such a way that the cross-linking isperformed after fabrication and application of the layer of electrolyticmaterial.

For example, with a polyvalent cation, a sufficiently weak concentrationassociated with the presence of a volatile acid prevents substantialcross-linking as long as the acid has not been eliminated byevaporation. The cross-linking of the resin reduces contact adhesioncapacity of the layer of electrolytic material. According to a preferredstructure of the layer of electrolytic material, a cross-linked layer,mechanically solid and coherent but non-adhesive, is associated with oneor two external layers which are less solid but adhesive and fabricatedto comprise a suitable non-cross-linked resin.

The electrolytic material can further comprise a complexing agent (suchas, for example, tartaric acid, citric acid, the oxalate anion) whichcan assist in solubilizing of certain metallic salts and/or facilitatingco-deposition of several ions into a metallic alloy.

The electrolytic material can further comprise one or more compounds,substances, dissolved or dispersed constituents which are capable ofimproving various characteristics. The characteristics which can beimproved thereby include stability of the electrolyte material and/orits optical, mechanical and electrical properties, and/or the appearanceand/or other characteristics of picture elements, reversibility of thewrite-erase process, number of accessible cycles without degradation,writing and erase speeds, "memory" (or, nonvolatility of the writing),electrical writing threshold voltage and the electro-opticalcharacteristics.

The electrolytic material can further comprise one or more agents forapplication in a layer of small thickness of the electrolytic material.The agents may be, for example, surface-active agents, plastifyingsubstances in particular.

The electrolytic material can possibly include residues of preparation,application or conservation agents belonging to a particular method ofmanufacture or application as a layer or film on the electrolyticmaterial or of a formative composition of the electrolytic material.

A particular preferred process for the manufacture of the electrolyticmaterial and its use as layers or films of small thickness in elementarylight-modulating cells comprises the non-limiting steps of fabricating afluid formative composition comprising at least the constituents of theelectrolytic material and additional water (the latter in a quantitysuch that the fluid formative composition has an appropriate fluidityfor the application or formation in a layer on at least one of theelectrodes of an elementary light-modulating cell), and possiblyprocessing (in particular drying, heat treatment) until a solidconsistency is obtained.

"Fluid formative composition" is understood to mean a composition havingthe properties of a fluid material (either spontaneously, or under theeffect of externally applied stresses such as those necessary for itsapplication in a layer). It may be possible to fabricate theelectrolytic material in its solid consistency spontaneously byinterruption of the external stresses, by evaporation of an excess ofwater and/or of volatile substances, or by induction of a solidconsistency by various means and processes. The fluid formativecomposition also comprises the electrolytic material in its finalcomposition as long as it remains deformable without rupture in acontinuous way in the presence of externally applied stresses.

A fluid formative composition of the electrolytic material is obtainedby dissolution of water-soluble constituents and dispersion ofconstituents which are not soluble in water. This dispersion and/ordissolution may possibly be followed by evaporation of part of thiswater (or by dilution by means of an adding water until the appropriateviscosity is obtained).

The formative composition can also contain substances intended tofacilitate its application or formation in a layer, such as, forexample, surface-active agents and plastifying substances. It can alsocontain substances intended to maintain stability of the formativecomposition and/or enable its continuous deformability (that is, withoutrupture). For example, in the presence of a cross-linking agent intendedto cross-link the resin in the layer of electrolytic material, the fluidformative composition can contain cross-linking retarding agents, suchas, in a nonlimiting example, complexing agents. To prevent thehydrolysis of the soluble salts in the case of high dilution, it can,for example, contain acids, in particular volatile acids.

Such substances can be partially or totally eliminated from the layer ofelectrolytic material, for example, by evaporation if they are volatile.Or, on the contrary, they may remain in the layer of the material.

A remarkable characteristic of this possible mode of manufacture of theelectrolytic material is that it allows a convenient adjustment of theviscosity of the formative composition over a wide range, from that of aliquid similar to water to that of a solid paste in the absence ofexternal stresses. It is also possible to adjust the viscosity to aconvenient value for the chosen mode of application or formation in alayer of small thickness. The mode of application or formation can bechosen from known techniques of application or formation in layers, suchas silk screening, air gap, helical wire bar (known as "coating bar"),scraper, extrusion and immersion in particular, and, more generally, allof the so-called "thick film" techniques.

The formative composition is applied in a layer which is preferablybetween a few microns and a few hundreds of microns thick, depending inparticular on its water content, in order to obtain a layer ofelectrolytic material. The layer of electrolytic material is of athickness preferably between a few microns and a few tens of microns,and is disposed over at least one of the electrodes of the elementarylight-modulating cell. It is possibly dried (by hot air, infra-red, andexposure to the ambient atmosphere in particular) until a material ofsolid consistency is obtained in its final composition. It can also besubject, possibly, to additional processing. For example, a heattreatment may be applied in order to obtain or accelerate cross-linkingof the resin.

The layer of electrolytic material can be used in elementarylight-modulating cells by applying or forming this layer in contact withone of the electrodes, followed by applying the other electrode to thefree face of the layer (FIG. 7A). With the preferred method of producingthe electrolytic material which provides it with a tack orpressure-sensitive adhesion or superficial adhesion, the cohesion of thecell is provided by simple adherence of the layer of electrolyticmaterial to each of the two electrodes. In addition to the adhesionprocured by the surface adhesion or the pressure-sensitive adhesion ofthe electrolytic material (and even in its absence), the directformation of the material in a layer on a substrate (such as one of theelectrodes) from a fluid formative composition which is spread and thendried can provide a natural adherence to this substrate. This naturaladherence can be much stronger than that of a layer of material which isfirst formed independently and then subsequently made to adhere.

The fluid formative composition can be spread or applied to an electrodein a single layer or in several consecutive layers with intermediate orsimultaneous dryings. The different layers can be identical to eachother, but it is also possible to fabricate each layer with a differentpercentage of the total constituents, all of the layers containing themall.

It is also possible to apply consecutive layers of electrolytic materialhaving different compositions. In this way, a layer of electrolyticmaterial having a composite structure is obtained. For example, it ispossible to fabricate a lower layer (that is, directly applied to thesubstrate) with a water-soluble resin which is easily crosslinked. Suchresins may be, for example, sodium carboxymethylcellulose (and areticulation agent). It is possible to fabricate the upper layer with aresin providing a tack or a pressure-sensitive adhesion (such as, forexample, hydroxyethylcellulose, polyvinylpyrrolidone, polyvinyl alcohol,and so forth).

The composite layer of electrolytic material thus formed adheres to theelectrode on which it has been formed, has high solidity, and exhibits apressure-sensitive adhesion which enables (in a possible mode ofconstruction of the elementary modulating cell) construction of the cellby applying the second electrode to the free adhesive surface of thecomposite layer to which it adheres. It is also possible to apply afirst layer (simple or composite) of electrolytic material to one of theelectrodes, and to apply a second layer (simple or composite) to thesecond electrode, and to fabricate the elementary cell by joining of thetwo layers into a single composite layer by placing the two layers incontact with each other by their free faces. With at least one of thetwo layers formed according to the mode which provides apressure-sensitive adhesion or tack, the cohesion of the cell is ensuredsolely by the adherence of the layers of materials to the electrodes andto each other.

Instead of fabricating continuous layers with the electrolytic material,it is also possible to divide it into granules or particles and toconstruct the layers by juxtaposition of such granules, either alone ormaintained, for example, by means of a binder.

In an independent device which is mainly a display device comprising amultiplicity of picture elements, the layer of electrolytic materialcan, in certain constructions, be common to all of the elementary cellsand occupy the entire surface of the screen. In other constructions ofthe device, it can be distributed according to a surface pattern ofportions of layer. That is, it may be divided into portions of a layeror layers of reduced area, the areas being independent and distributedover the screen, each belonging to an elementary cell or to a particularrestricted group of elementary cells (for example, common to the cellsof a same row or column in a matrix display device). Obtaining suchpatterns with high resolutions is particularly easy with the presentmethod of manufacturing layers of electrolytic material, in particularbecause of the mask, stencil and silk screen techniques used indepositing processes known as "thick film" processes.

It is also possible, in a variant of the process, to construct the layerof electrolytic material on a temporary substrate constructed from anon-adhering material such as polytetrafluoroethylene, and then transferit to one of the two electrodes.

A light-modulating cell comprises at least, in combination:

1) a first "working" electrode which is transparent or substantiallytransparent and electronically conducting;

2) a second "counter-electrode" separated transversely from the workingelectrode and electronically conductive;

3) at least one layer (or portion of a layer) of the previouslydescribed electrolytic material, interposed between the two electrodesand in contact with them in the region of a picture element; and

4) electrical connection zones for leading electrical current to theworking electrode and to the counter-electrode, capable of enabling (onthe one hand) application to the working electrode of a negativeelectrical voltage with respect to that of the counter-electrode, and(on the other hand) allowing passage between the electrodes of a currentwhose direction is opposite to that of the electrical current resultingfrom the application of the previous voltage.

An elementary light-modulating device comprises at least:

1) a first "working" electrode which is transparent or substantiallytransparent and electronically conductive, possibly supported by atransparent first substrate or front substrate;

2) a second "counter-electrode" or auxiliary electrode transverselyseparated from the working electrode and electronically conductive,transparent or substantially transparent if the elementary device isintended to function by transmission but without requiring transparencyif the elementary device is intended only to function by reflection,possibly supported by a second transparent substrate if the elementarydevice is intended to function by transmission;

3) at least one layer (or portion of a layer) of electrolytic materialsuch as previously described, interposed between the two electrodes andin contact with them in the region of a picture element;

4) electrical connection zones for leading electrical current to theelectrodes

(that is, elements 1, 2, 3, 4 comprise a modulating cell);

5) electrical current leads in contact with the electrical connectionzones;

6) electrical connections extending the current leads; and

7) at least one mechanical substrate to support the device.

An elementary device can further comprise means of contrasting and/ormasking the periphery of the optically densifiable zone (pictureelement), if these means are not already intrinsically created by thecomponents of the elementary device. An elementary device can alsocomprise means of insulating and protecting the components from ambientatmosphere, and/or means for maintaining the device's cohesion and/or ofthe permanence of its internal electrical contacts.

An independent light-modulating device comprises at least one suchelementary device and, generally, may comprise a plurality of them(particularly in the case of a display device). Adding extensions or oneor more connectors (or one or more electrical connection zones) to theelementary devices and adding mechanical supporting means provide thedevice with structural rigidity, the whole assembly enabling use of thisdevice as an independent unit.

The complementary components of the independent device are inparticular: mechanical supports or substrates, casing, encapsulation,internal connections, connector(s) or connection zone(s), and printedcircuit boards, as already described. In the particular case of anindependent device comprising a plurality or multiplicity of elementarydevices, the extrinsic components or constituents of the differentelementary devices can be merged and/or combined.

Such an independent light-modulating device, in particular a displaydevice, comprises only solid materials as a consequence of thepreviously explained properties of the electrolytic material. It has aremarkable simplicity of structure and construction, with reducedrequirements with regard to constituent materials and components, and avery wide tolerance with respect to external stresses. This enablessimple and inexpensive construction of light-modulating devices and, inparticular, of various display devices with direct or matrix addressing,especially very large display panels (such as, for example, forstadiums, airports, and so forth).

In fact, it is not necessary to maintain a strict parallelism and anaccurate spacing between the electrode and the counter-electrode of anelementary display device. Consequently it is unnecessary to provide aspecific spacer; a layer of electrolytic material such as deposited bythe known industrial techniques of coating or application suffices tofabricate the spacer and to define a separation of sufficient accuracyand parallelism.

It is no longer necessary to use a working electrode and acounter-electrode having perfect flatness. The deformability, or plasticor viscoelastic compliance, of the electrolytic material enables it toclosely follow a general curvature if it exists, as well as followinglocal defects. This ensures excellent physical and electrical contactnecessary for functioning. For example, it is thus that transparentelectrodes deposited on an ordinary drawn glass plate are suitable forproducing display devices.

It is no longer necessary, with the present electrolytic material (whichprovides a tack or a pressure-sensitive adhesion) to provide specificmechanical means for maintaining the cohesion of the elementary displaycell.

Nor is it necessary to provide specific mechanical means for applyingand maintaining a pressure on the cell for providing and conservingexcellent physical and electrical contact necessary at each interfacebetween the electronic conductor and ionic conductor, for correctfunctioning. The 15 adherence of the layer of electrolytic material toeach electrode suffices to maintain cohesion of the elementary cell andthe quality and permanence of the electrical contact.

Finally, with the method of producing the electrolytic material whichenables it to maintain permanent conductivity even when in contact withthe ambient atmosphere, it is not necessary to provide strictlyleak-tight means of sealing the elementary cell or device to completelyprevent the entry of air and/or atmospheric humidity and release ofhumidity contained in the layer of electrolytic material. A protectiveinsulation of the device is generally desirable or even necessary in thecase of functioning in extreme atmospheric environments which arecorrosive or aggressive, in order to limit or prevent the contaminantsand possible corrosive agents present in the external medium fromgaining access to the components of the device, including those externalto the cells (such as, for example, the connecting conductors). But,generally, this may be accomplished without having to go up to theconstraints of a leak-tight sealing of each cell.

With regard to devices such as very large-area display panels, it isknown that these can be naturally subjected to a set of stresses capableof effecting their physical integrity and correct functioning. Becauseof this large size, differential thermal expansions (for example, thoseresulting from the unilateral or partial exposure to the sun or from theasymmetric proximity of a heat source), flexions (for example, thoseresulting from the effect of wind or mechanical stresses imposed by thesupporting architecture or framing), and vibrations (for example, thoseinduced by passing traffic or by shocks), for example, give rise tolocal strains and stresses which can be considerable. They are capableof altering the physical integrity of the modulating cells, degradingthe quality of the electrical contact at interfaces, and altering theleak-tightness of the seals which are essential in known (especiallyelectrochromic) display devices.

On the other hand, in the present devices, the combination of propertiesof plastic or viscoelastic deformability, of pressure-sensitive adhesionof the electrolytic material, and the absence of the necessity ofstrictly leak-tight sealing, enables large panels not to be affected intheir integrity or functioning by the abovementioned thermal andmechanical stresses.

Such a device thus generally described is now described in more detail.

The working electrode comprises a material having properties ofelectronic conduction and substantial optical transparency. It may be,for example, a thin layer, generally of a few tens to a few thousandAngstroms thickness, of gold, tin oxide ("TO"), indium oxide, mixedoxide of tin and indium ("ITO"), or equivalent. This list is not to beconstrued as limiting, and its entries are solely indicative.

Such a layer is generally deposited on a transparent substrate such as aglass plate or a sheet of plastic material which can then constitute afront substrate of the elementary device. It can even constitute asingle front substrate for all the individual working electrodes of anindependent device when it comprises a multiplicity of picture elements.In this case, the individual working electrodes comprise a pattern ofportions of thin transparent conductive layer deposited on such a singlefront substrate or are patterned by selective etching of a single layer.The "NESA" glass produced by PPG INDUSTRIES, comprising a thin layer oftin oxide deposited on a glass plate, is an example of a transparentelectrode and a substrate which can be used.

The counter-electrode is fabricated like the working electrode from aconductive and transparent material if the light-modulating device isintended to function by transmission or by transparency. If it is a thinlayer deposited on a transparent substrate of glass or plastic material,the latter can constitute a single back substrate of all of thecounter-electrodes of a modulating device when the latter comprises amultiplicity of picture elements. In this case, in the same way as theindividual working electrodes, the individual counter-electrodescomprise a pattern of portions of thin transparent conductive layerdeposited on such a single back substrate, or are patterned by selectiveetching of a single layer.

However, if the modulating device already comprises a singlefront-substrate capable of forming a mechanical support for the device,it can be advantageous (particularly in order not to introduce possibleadditional rigidity) to have counter-electrodes mechanically independentfrom each other (and consequently, not to constitute a single backsubstrate). The counter-electrode does not need to be transparent if thelight-modulating device is intended to function by reflection. It isthen sufficient for it to have electronic conduction properties. A verylarge number of homogeneous or composite materials having electronicconduction are suitable. It is advantageous to use counter-electrodematerials in the form of sheets and layers of small thickness, andpreferably having, in this form, a certain flexibility or deformability.

"Counter-electrode material" is understood to mean a homogeneous orcomposite material from which it is possible to fabricate a surfacepattern of counter-electrodes. Such a material can be, for example, aflexible sheet of pyrolytic graphite, a plastic material filled withcarbon particles or metal particles, a conductive paste for silkscreening, or a sheet of plastic or glass material of which one face iscovered with a thin layer of a transparent semiconductor oxide.

For example, it is possible to use as a counter-electrode material, thinmetallic sheets, flexible sheets of pure graphite (the "Grafoil"flexible sheets by Union Carbide or the "Papyex" flexible sheets byCarbone Lorraine, and so forth), graphite or carbon sheets or fabrics("RVC" carbon felts and "RVG" graphite felts carbon fabric "TCH" andgraphite fabric "TGM" by Carbone Lorraine, and so forth). This list isnot to be construed as limiting the invention, and its elements aresolely indicative.

It is also possible to use composite conductor materials such as sheetsof plastic or elastomer materials (polyvinyl chloride, polyolefins,silicones, and so forth) filled with particles, fibers or flakes ofsubstances having electronic conductivity, for example, metals: copper,silver and nickel in particular (such as the "Conmax" by Tecknit filledwith nickel in particular), semiconductors: tin oxide, indium oxide inparticular, graphite or carbon (such as "Condulon" sheets by PervelIndustries, "Cabelec" by Cabot, "Abbey 100" by Abbey PlasticsCorporation in particular). This list is also not to be construed aslimiting the invention, and its elements are purely indicative.

It is also possible to use as a counter-electrode material an originallyfluid conductive composition, generally composed of a resin and aparticulate electronic conductive filler and possibly a solvent,deposited on a substrate and then dried or polymerized, for example, anink or a conductive paste which can be silk-screened (such as thegraphite-based "Electrodag 423 55" by Acheson, the copper-based"ACP-020J" and the graphite based "TU-40S" by Asahi Chemical, and soforth), or, for example, a conductive varnish or conductive paint (suchas the copper-based "Copalex 100" by Showa Denko, the nickel based"Electrodag 440AS" and the graphite-based "Electrodag 5513" by Acheson,the silver-based "Acrylic-I" and the carbon-based "Latex 1000" byTecknit, and so forth) deposited in a thin layer by known techniques ofsilkscreening, gun spraying, coating by air gap or coating bar,immersion, and so forth. (The so-called "thick film" techniques areunderstood to mean all of these techniques and various techniques forthe formation of a film or solid layer in thicknesses of the order ofthose previously defined as being of "small thickness" from a fluidmaterial.)

In this case it is possible to use one of the previously quotedconductive materials in the form of a sheet, and thus to fabricatecounter-electrodes having composite structure provided by theassociation, for example, in superimposed layers, of several differentcounter-electrode materials, an association which is possibly capable ofadvantageously combining their characteristics. It is also possible inthis case to use an electrically insulating substrate, impervious oralternatively porous or alternatively perforated, for example, a thinfilm of electrically insulating plastic material which is impervious orperforated or a sheet of a non-woven synthetic fiber material. Thisarrangement can advantageously be implemented in a display devicecomprising a multiplicity of picture elements. It enables on a singleback insulating substrate (preferably in the form of a thin flexiblesheet) to simply and economically produce the entire pattern ofcounter-electrodes using the thick film techniques mentioned above.

This single insulating back substrate can be permanent (that is,constitute a definitive component of the display device). It can also,by an appropriate choice as a sheet of non-adhesive plastic materialand/or of a sheet covered with a layer of coating release material, bepresent only temporarily, to subsequently be removed once the device iscompleted. In this case, it only constitutes a convenient intermediatemeans of manufacture, enabling advantageous production of the pattern ofcounter-electrodes.

It is also possible, in another embodiment of a counter-electrode of adevice, to deposit an originally fluid conductive composition usingthick film techniques directly on the layer of electrolytic material,which is itself possibly already previously applied to the workingelectrode. In a display device comprising a multiplicity of pictureelements, it is thus possible to directly apply (using thick filmtechniques and using appropriate screens, stencils and/or masks) adesired pattern of portions of the counter-electrodes layer on acoordinated pattern of portions of the electrolytic material layer whichis itself already applied on another coordinated pattern of workingelectrodes deposited on a transparent substrate which can constitute amechanical support for the whole of the display device. It can even becapable of constituting the sole mechanical support of the device.

The above portions of the layer or layers of reduced area forming acounter-electrode are portions of layers which are independent from eachother and which can, depending on the case, each belong to an elementarycell or can each be common to a particular restricted group ofelementary cells. For example, they may be common to the cells of a samerow or a same column.

The layer of electrolytic material can be used in the light-modulatingdevice by application or formation of a layer 3 in contact with one ofthe two electrodes according to one of the described processes, followedby the application of the other electrode in contact with the free faceof the layer of material. As already mentioned, with the preferredmethod of producing the electrolytic material which provides it with atack or a pressure-sensitive adhesion or superficial adhesion, it is notessential to provide specific mechanical means for maintaining thecohesion of the elementary cell thus fabricated. Nor is it essential toprovide and conserve an excellent physical and electrical contact at theinterface between each electrode and layer of electrolytic material, theadherence of the layer of electrolytic material to each electrode beingsufficient for this purpose.

With this method of producing the electrolytic material, the latter canalso be implemented by application or formation of a first layer incontact with one of the two electrodes, and a second layer in contactwith the other electrode, followed by the joining the two layers into asingle composite layer by placing in contact and adhesion with eachother the free faces of the two layers.

In a display device comprising a multiplicity of picture elements, thelayer of electrolytic material can be common to all elementary displaycells, and can occupy the entire surface of the screen. It can also bedistributed according to a surface pattern (that is, divided intoportions of layer or layers of reduced area which are independent anddistributed over the screen, each belonging to an elementary cell oreach common only to a restricted number of elementary cells, forexample, common to the cells of a same row or a same column in a matrixdisplay device). Obtaining such surface patterns is particularly easywith the described electrolytic material due to the techniques of masks,stencils, screens, and so forth, used in the "thick film" depositingprocesses.

The means of electrical connection inside the independent device can bechosen from among all of the electrical linking or connection processeswhich can be used.

"Electrical connection material" means a material which is homogeneousor composite from which it is possible to fabricate one or more surfacepatterns of electrical connections connecting the elementary cells tothe connector(s) or connection zone(s) to which the power supply andelectronic control and addressing circuits must be connected. Such amaterial can, for example, be a silver or copper paste for silkscreening, a conductive lacquer, a selfadhesive copper strip or aprinted circuit conductor in particular, the list not intended to limitthe invention, its elements being solely indicative.

It can be advantageous to use conductive inks, pastes or lacquers,particularly based on particulate silver (such as, for example,"Electrodag 1415" and "Electrodag 427 SS" by Acheson, "CON/RTV-I" byTecknit, "LS-400" by Asahi Chemical, "L 2003" and "L 2030" by Demetron,and so forth) which, implemented by thick film techniques, convenientlyenable the establishment of electrical contact of very good quality withboth the working electrode and the counter-electrode. It alsoestablishes an electrical bond or "weld" (that is, a means forelectrical connection) between one or another electrode and an internalconnecting conductor such as a metal wire, copper strip, printed circuitboard conductor, or conductive ink or paste. The latter morespecifically enable the construction of a connecting conductor which canbe conformed to any relief and path whatsoever (such as those imposed bythe back surface of a display device comprising a multiplicity ofpicture elements to which it is advantageous to apply). It furthermoreenables it, by an appropriate choice of ink, to be deformable withoutrupture. Such connecting conductors enable each electrode to beconnected in a very practical way to a metallic conductor or to a rigidassembly of metallic conductors (such as a printed circuit board distantfrom the 15 electrodes to be connected), or to connect each electrode toa conductor or connection zone of the device (for example, at one edge,from which it can be more convenient to connect the device electricallyto the addressing and control electronics).

Means for masking the periphery of the picture element, (that is, in thecase of a display device comprising a multiplicity of picture elements,of the interstitial space filling the entire screen surface with theexception of the picture elements themselves) can be necessary forconcealing the internal connecting conductors and all other elements ofthe structure of the display device which could be visible. It can alsobe necessary, as simultaneous means of contrast, for contributing to thecontrast of the written picture elements with respect to the remainingpart of the screen, this remaining part comprising the non-writtenpicture elements and this interstitial space.

In the case, for example, of a display device operating solely byreflection, the picture element appears black when it is written andwhite (because of, for example, a contrasting white pigment present inthe electrolytic material) when it is erased (that is, not written). Itis then desirable, in a first method of display (which can be brieflysummarized as the display of black images on a white background) thatthe means for masking the periphery of the image picture element shouldhave a white appearance which is as close as possible to the white ofthe non-written picture elements, which contributes to emphasizing theblack of the written picture elements in comparison with theirenvironment comprising all the non-written picture elements and theperipheral interstitial space, thus maximizing contrast.

In a second method of display which can be briefly summarized as thedisplay of white images on a black background, it is converselydesirable that the means for masking the periphery should have a blackappearance as close as possible to the black of the written pictureelements. This contributes to emphasizing the white of the non-writtenpicture elements in comparison with their environment comprising all thewritten picture elements and by the peripheral interstitial space, hereagain maximizing contrast.

In the case of a display device functioning by transmission (that is,back lit), it is generally preferable to minimize reflection of ambientlight by the periphery of the picture element which degrades thecontrast, and consequently, to give to this periphery a black appearanceor at least a dark appearance and as little reflecting as possible.

In the case of display devices functioning by reflection, the means ofmasking and contrast of the periphery of the picture element are alreadyintrinsically created when the layer of electrolytic material is givento the whole extent of the screen surface: the masking and contrastingpigment present in the layer of electrolytic material applies its actionover the entire screen surface.

If it is not desired or if it is not possible, for a device functioningby reflection, to give to the layer of electrolytic material an extentcovering the entire screen surface (for example, resulting from thechoice of a manufacturing method of the device) which does not permitit, or, if the device is intended to function by transmission (andtherefore uses a transparent electrolytic material), it is appropriateto implement specific masking and contrasting means for the periphery ofthe picture element.

Such masking and contrasting means can comprise, for example,application to this periphery (prior to the positioning of the means ofconnection and all of the components to be masked) a layer of maskingand contrasting 10 material such as a layer of paint, ink, varnish,polymer or elastomer containing pigments and/or colorants in quantitiessuch that a layer of sufficient thickness constitutes an opaque maskexhibiting the desired color. Very numerous materials in thesecategories can be suitable, particularly those capable of drying orhardening or cross-linking at ordinary or slightly raised temperatures.Suitable materials with these characteristics include, for example,paints, lacquers or cellulose, vinyl, acrylic varnishes, and inparticular colored inks and pastes for silk screening, single-componentpigmented silicone elastomers reticulating at ambient temperature (suchas the "Rhodorsil CAF" by Rhone-Poulenc, in particular), pigmentedtwo-component resin-hardener or resin-catalyst mixtures (epoxy,silicones in particular) polymerizing or vulcanizing at ambienttemperature or at a temperature close to ambient temperature inparticular. Such materials are commercially available in the form ofsuspensions and/or solutions in an appropriate solvent or in the form ofa not cross-linked monomer fluid as a single component or as twocomponents to be mixed shortly before use.

They can be applied in a regular coat on the back side of the device,through an appropriate pattern of masks, screens, stencils, and soforth, intended to protect the zones which must not be covered (forexample, the points, lines or zones of the working electrodes andcounter-electrodes where electrical contacts must be made in the case inwhich the electrical connections are established after masking) or whichmust not be masked (for example, the transparent counter-electrodes onthe portion of their area corresponding to the picture element itself inthe case of a device functioning by transmission). This may be achievedparticularly with the help of all known appropriate applicationtechniques: gun spraying, silk screening, immersion, coating by roller,and printing techniques in particular.

Means of insulation and protection of the components of the device fromthe external environment are generally desirable or even necessary toprevent contact between these contaminant components and/or corrosiveagents present in the external atmospheric environment, and possibly toprotect them from rain, fog, and various accidental projections, andpossibly from shocks. In extreme cases it may be considered necessary togive these means of insulation and protection a strict degree ofimperviousness to liquids, gases, or any other substances whosepenetration into or exit from the device is not desired.

Such means of insulation or protection can, for example, comprise anadditional layer of paint, varnish or resin, particularly such asdescribed previously as masking and contrasting means applied using theabove-mentioned techniques over the entire rear face of the device ifsuch a layer has a sufficient imperviousness to liquids and gases. Suchmeans can, more generally, comprise the coating, potting or impregnationof the device to be protected and, in particular, of the rear face bymeans of a polymer or an elastomer available in the form of a fluidmonomer with an added hardener or reticulation catalyst, a suspension orsolution of resin in a liquid or appropriate solvent, able (notnecessarily) to contain a filler.

In particular it can be advantageous to cover, embed or impregnate therear of the device starting from its edges with a potting or sealingresin of the type used for potting printed circuits, for example, resinswhich are adhering and preferably flexible after cross-linking, such assilicone elastomers (for example, "RTV" by General Electric, and soforth). Such a polymer is preferably colorless and transparent if thedevice is intended to function by transmission (for example, the "RTV615" silicone rubber by General Electric). Such a resin which istransparent and has good optical properties may also serve for totallypotting such a device, whether it functions by transmission or byreflection, providing it with maximum protection.

Even though specific means of cohesion of cells are not essential in thecase in which this cohesion is provided by the pressure-sensitiveadhesion properties of the electrolytic material, the means of maskingand the above means of insulation and protection can contribute tocohesion.

A picture element is determined in shape, size and position by theintersection of orthogonal projections, on the screen surface, of theareas of the first electrode, the counter-electrode and the layer ofelectrolytic material of each elementary display cell.

The possibility of defining a picture element simply by such anintersection results from the above revelation that (in the describedlight-modulating process, and with components having the indicatedthicknesses) the picture element is the area corresponding to such anintersection. The area is delimited by a remarkably sharp contour, eventhough it is formed at a distance from the counter-electrode equal tothe thickness of the layer of electrolytic material. While it could havebeen expected to have a blurred or diffused contour, at least for theportion of the contour determined by the counter-electrode, the increasein optical density, once formed, neither diffuses nor is diluted at theperiphery of the picture element.

Advantage is taken of this possibility for the economic construction ofdisplay devices according to a possible embodiment comprising amultiplicity of picture elements, in particular, very high-resolutiondot matrix devices. (This embodiment is denoted below by "matrixpanel".)

According to this embodiment, each of the elementary display cells ismainly described as the superposition of three layers or films(electrode, layer of electrolytic material, counter-electrode). Each ofthese three principal components is able to serve to define a portion orthe totality of the contour of each picture element, without it beingnecessary to make use of other means and/or other components. Eachcomponent of such an elementary cell only functions or operates withinthe contour of the picture element. Any extension of this componentbeyond this contour can, if necessary, be used in another adjacentelementary cell without it being necessary to provide a substantial gapbetween the two zones of the component other than that corresponding tothe relative geometry of these two adjacent image points.

It is not essential to limit the minimum dimension of the pictureelement to allow for a diffused or blurred contour and arrange for theexistence of a sufficient zone of maximum optical density. Each pictureelement can be as small as the application or patterning techniques ofthe films or layers constituting the components allow.

It is not essential to provide a substantial minimum separation or gapbetween adjacent picture elements in order to avoid inter-penetration ofthese picture elements due to a diffused or blurred contour which wouldhave the effect of reducing resolution and contrast.

Neither is it essential to provide a substantial minimum gap betweenadjacent picture elements to avoid an interpenetration of these pictureelements which would be due to diffusion or dilution of opticaldensification of a picture element within its periphery (that is, in theadjacent picture elements).

Finally, it is no longer essential to provide, in order to maintain theoptical densification of a picture element at its contour, additionalparticular means of confinement of the picture element within adetermined contour which would increase the area occupied by theindividual cell and consequently would increase the gap between adjacentpicture elements and, furthermore, would increase the complexity andcost of the display device.

On the screen area of a display device comprising a multiplicity ofpicture elements, a "surface pattern of picture elements" is understoodto mean the surface geometric pattern of the individually addressablepicture elements desired, such that the optical densification (that is,the darkening or opacification of selective combinations of thesepicture elements) can represent alphanumerical characters, images andother graphical arrangements which are to be displayed. A surfacepattern of picture elements of a display device corresponds to a spatialdistribution in the device of elementary display cells whose componentsare (in the described display devices) superimposed layers or films ofdefined shape and area. The shapes' orthogonal projection on the screenarea includes at least the picture elements.

Each elementary cell can comprise its own individualized components. Butit is possible that similar components of a given type are separatezones of a single component which is common to some or all of theelementary cells. For instance, in a dot matrix type display device, thetransparent electrodes of a same column of pixels can be part of asingle transparent conductive electrode in the shape of a strip, commonto all of the elementary cells of the column.

On a display device screen comprising a multiplicity of pictureelements, "surface pattern of components of the same type" is understoodto mean the geometric pattern formed by all of the components of theelementary display cells belonging to this same type of component, eachof such components being able to belong to one cell or to be common to agroup of cells.

"Intersection of several superimposed surface pattern" is understood tomean the geometric pattern formed by common areas of the orthogonalprojections on the screen area of the various surface patternsconcerned.

For linguistic convenience, it is appropriate that the expression"common to a group of cells or elementary devices" and applied to theterm working electrode or counter-electrode or layer of material (orportions of each of them) applies when the working electrodes,counter-electrodes and the layers of material respectively of the groupof cells or devices concerned are mutually distinct zones of thecomponent referred to as common to this group.

"Coordinated (or conjugated) surface patterns" of components of adisplay device is understood to mean superimposed surface patterns ofcomponents of each type, such that the association of the componentsdetermines as many complete elementary display cells as the number ofpicture elements the display device must comprise, and such that thespatial distribution and size of these elementary cells are compatiblewith the desired locations and dimensions of the picture elements on thescreen of the device.

In order to construct, according to this embodiment, a display devicecomprising a multiplicity of picture elements, there is associated bysuperimposing (without using other means for assisting in thedelimitation or separation of the picture elements) three surfacepatterns of components patterned and coordinated in such a way thattheir intersection defines the desired surface pattern of individuallyaddressable picture elements. The surface patterns comprise one surfacepattern of transparent working electrodes, one surface pattern ofportions of layer of electrolytic material, and one surface pattern ofcounter-electrodes, in this order.

By convention, the term "pattern" has been reserved in the rest of thedescription to all possible cases, with the exception of those in whichthe component concerned is single and common to all of the elementarycells of the device, in which case the qualifying expression "single andcommon to the entire screen" applied to the component concerned, isused.

According to this embodiment of a display device comprising amultiplicity of picture elements, there are numerous possibilities ofstructure and construction. Each component of different type can be usedfor defining a portion or the entire contour of picture element asdescribed above. It is possible to choose, for a given display device,the most advantageous combination of patterns from the point of view ofsimplicity of manufacture of the elementary cells and internalelectrical connections of the device. According to the types ofcomponents chosen for distribution over the screen according to asurface pattern and those chosen to retain as a single component commonto all of the elementary cells of the screen, there are, in particular,the following combinations:

First variant: Working electrodes 2 alone are distributed according to asurface pattern, counter-electrode 4 and layer of electrolytic material3 being single and common to the entire screen. This is an arrangementwhich can be used in particular for direct addressed devices (FIG. 8A).

Second variant: Counter-electrodes 4 alone are distributed according toa surface pattern, working electrode 2 and layer of electrolyticmaterial 3 being single and common to the entire screen. This is anarrangement which can be used in particular for direct addressed devices(FIG. 8B).

Third variant: Working electrodes 2 and portions of electrolyticmaterial 3 distributed according to coordinated surface patterns,counter-electrode 4 being single and common to the entire screen. Thisis an arrangement which can be used in particular for direct addresseddevices (FIG. 8C).

Fourth variant: Counter-electrodes 4 and portions of layer ofelectrolytic material 3 are distributed according to coordinated surfacepatterns, working electrode 2 being single and common to the entirescreen. This is an arrangement which can be used in particular fordirect addressed devices (FIG. 8D).

Fifth variant: Working electrodes 2 and counter-electrodes 4 aredistributed according to coordinated surface patterns, layer ofelectrolytic material 3 being single and common to the entire screen.This is an arrangement which can be used in particular for directaddressed devices or for matrix addressed devices (FIG. 8E).

Sixth variant: Working electrode 2, counter-electrodes 4 and portions oflayer of electrolytic material 3 are distributed according to threecoordinated surface patterns. This is an arrangement which can be usedin particular for direct addressed devices or for matrix addresseddevices (FIG. 8F).

Furthermore, in the last four variants, surface patterns of two types ofcomponents can advantageously be identical or substantially identical,(that is, they may be merged or substantially merged, in numerous caseswhere this merging is likely to constitute an advantage, for example,from the point of view of manufacture).

The following possibilities can thus be considered:

Sub-variant of the third variant: Merged patterns of working electrodes2 and portions of layer of electrolytic material 3, counter-electrode 4single and common to the entire screen.

Sub-variant of the fourth variant: Merged patterns of counter-electrodes4 and portions of layer of electrolytic material 3, working electrode 2single and common to the entire screen.

Sub-variant of the fifth variant: Merged patterns of working electrodes2 and counter-electrodes 4, layer of electrolytic material 3 common tothe entire screen.

First sub-variant of the sixth variant: Merged patterns ofcounter-electrodes and portions of a layer of electrolytic material 3,different coordinated (conjugated) patterns of working electrodes (FIG.8G).

Second sub-variant of the sixth variant: Merged patterns of workingelectrodes 2 and of layers of electrolytic material 3, differentcoordinated patterns of counter-electrodes 4 (FIG. 8H).

Third sub-variant of the sixth variant: Merged patterns of workingelectrodes 2 and of counter-electrodes 4, different coordinated patternof portions of layer of electrolytic material 3 (FIG. 8I).

Fourth sub-variant of the sixth variant: Merged patterns of the threetypes of components (FIG. 8F).

All of these variants of particular combinations of structures can beproduced by the first method of manufacture described below. Thesub-variants of the fourth variant and the first and fourth sub-variantsof the sixth variant can also be produced by the second manufacturingprocess, also described.

"Associated pattern of a material, component or constituent" isunderstood to mean an appropriate geometric and suitably associatedpattern, connected or combined with components of elementary cells of alight-modulating device comprising a plurality of cells, of a material,component or peripheral constituent, extrinsic or specific, necessaryfor the functioning of the elementary cells and/or enabling the use ofthe device as an independent unit.

A first possible method for manufacturing a light-modulating devicecomprising a multiplicity of picture elements, particularly applicableto the preferred structure, which lends itself to industrial manufactureat low cost price, comprises applying (using known "thick film"techniques such as the silk screening techniques) a layer ofelectrolytic material and a layer of counter-electrode material, eachdivided into portions of layer defined and distributed according tosurface patterns coordinated with a surface pattern of transparentelectrodes, in order, in particular, that the intersection of the areasdefined by the three patterns determines the desired surface pattern ofpicture elements.

It also comprises possibly applying (using the same thick filmtechniques) associated patterns of connection and/or insulatingmaterials capable of being implemented using the above-mentionedtechniques, and forming the network of internal electrical connectionsof the device or at least part of the latter.

It also comprises possibly applying (using the same thick filmtechniques) associated patterns of masking and contrasting material,insulation, protection, impregnation or potting material, as well as anyother material used in the construction of a light-modulating device andcapable of being used by these techniques.

Use of layers of electrolytic material implemented by thick filmtechniques (such as, in particular, the silk screen techniques),combined with use of inks, pastes, lacquers, varnishes and resins (inparticular conductive, insulating and pigmented), in particularimplemented by the same techniques in order to produce thecounter-electrodes, current leads, electrical connections, contrastingmasks, coatings, pottings or protective impregnations or insulation,enables implementation of the first method of manufacture of displaydevices comprising a multiplicity of picture elements.

According to this process, there are formed (on suitable supports orsubstrates, using thick film techniques and using screens, stencilsand/or appropriate masks) coordinated surface patterns and associatedsurface patterns of the various above-mentioned components, materialsand constituents. And, by these means, there is manufactured at leastpart of the independent device comprising at least the totality of theelementary light-modulating cells and possibly at least part of theperipheral and extrinsic components of the cells and of the specificcomponents and constituents of the independent device.

According to a first variant of this first method of manufacture, thereis produced at least part of the display device by applying insuperposition (using thick film techniques on a transparent substrateforming a mechanical support and comprising a surface pattern oftransparent working electrodes) at least the following:

a coordinated pattern of portions of a layer of electrolytic material;

a pattern coordinated with the previous ones, of portions of layer ofcounter-electrode material, these three patterns being in particularsuch that their intersections determines the desired pattern of pictureelements;

and, possibly:

an associated pattern of peripheral contrasting or masking material;

associated patterns of current leads, of connecting conductors and ofinsulating layers comprising at least part of the network of internalconnections of the device connecting the working electrodes and thecounter-electrodes of the elementary cells to a connection zone or zonesor to a connector or connectors, these patterns being combined in such away that the various conductors are insulated from each other and fromthe working electrodes and the counter-electrodes to which they do nothave to be connected;

associated patterns of layers or applications of protective, insulation,impregnation or potting materials.

The order of applications being able to be different from the orderdescribed above.

According to a second variant of this first method of manufacture of alight-modulating device of small thickness, there is produced at leastpart of the display device by applying in superposition (on a firstsubstrate, using thick film techniques such as, in particular, the silkscreening techniques):

at least one layer of counter-electrode material and one layer ofelectrolytic material, each divided into portions or defined zones anddistributed according to coordinated surface patterns, and possiblyassociated patterns of current leads, of connecting conductors and ofinsulating layers constituting at least part of the network of internalconnections of the device, associated patterns of masking andcontrasting material, of insulation, impregnation, protection or pottingmaterials, as well as any other material likely to be used in theconstruction of the light-modulating device and capable of beingimplemented by the above-mentioned techniques, and by applying in asingle movement or progressively by zones depending on the case thisfirst substrate at least already coated with some of its layers to asecond transparent and preferably rigid or substantially rigid substrateprovided with a surface pattern of transparent electrodes coordinatedwith the two previous coordinated patterns, these three patterns beingin particular such that their intersection determines the desiredpattern of picture elements. The application is made in such a way as toenable the maintaining by pressure, or by pressure-sensitive adhesion ofat least the assembly of layers of the first substrate in contact withthe second substrate.

According to this second variant there is produced at least part of thedisplay device by applying to a first substrate (using thick filmtechniques) at least the following:

a pattern of portions of layer of electrolytic material;

a pattern coordinated with the previous one of portions of layer ofcounter-electrode material;

and, possibly:

an associated pattern of peripheral masking or contrasting material;

associated patterns of current leads, connecting conductors andinsulating layers constituting at least part of the network of internalconnections of the device connecting the working electrodes and thecounter-electrodes of the elementary cells to a connection zone or zonesor a connector or connectors, these patterns being combined in such away that the various conductors are insulated from each other as well asfrom the working electrodes and counter-electrodes to which they do nothave to be connected;

associated patterns of layers or applications of protection, insulation,impregnation or potting materials, the order of applications being ableto be different from the order described above, and by applying thefirst substrate coated by at least some of the various above-describedpatterns to a second transparent substrate forming a mechanical supportand comprising a surface patterns of transparent working electrodescoordinated with those of the layers of electrolytic material and ofcounter-electrodes, these three patterns being, in particular, such thattheir intersection determines the desired pattern of picture elements,the application of the first substrate to the second being carried outin such a way as to enable to maintain by pressure or bypressure-sensitive adhesion, the whole formed by the patterns applied tothe first substrate in contact with the second substrate.

It is possible, in a first sub-variant of this second variant of thefirst method (in which the first substrate is preferably chosen from amaterial having anti-adhering properties or coated with a coatingrelease material) to perform the transfer onto the second substrate ofthe patterns coated or applied to the first substrate and to follow thisby removing the latter. Such a first substrate can appropriately be, forexample, a thin flexible sheet having an anti-adherent surface whichcan, once coated (that is, coated with its patterns), be completelyapplied to the second substrate in a single movement or by unrolling itand then removed, progressively separating the assembly formed by thesuperimposed configurations starting from an edge or a corner. Thisassembly is maintained in contact with the second substrate, preferablyby means of the pressure-sensitive adhesion properties of its concernedface, in particular resulting from the pressure-sensitive adhesion ofthe electrolytic material in a preferred composition. It can also, forexample, be a rigid cylinder which transfers all of its coated orapplied patterns to the second, generally flat substrate when it is madeto roll on the latter.

It is also possible, in a second sub-variant (in which the firstsubstrate is preferably a thin flexible insulating or insulated sheetwithout anti-adherent surface state) to allow (once this first substratecoated and applied to the second substrate) this first substrate toremain in position and then become a permanent component of the device.Such a first appropriate substrate can be, for example, a thin flexibleimpervious sheet of plastic material.

It is also possible, in a third sub-variant of the second variant of thefirst method of manufacture (in which the first substrate is preferablya thin flexible sheet without antiadherent surface state which has acertain porosity distributed or localized according to a distribution)to apply part of the coatings on the front side (that is, on the facewhich must be applied to the second substrate) and another part on thereverse side of this first substrate, the electrical connectionsnecessary between the layers located on either side of the firstsubstrate being made through the porosity. Such an appropriate firstsubstrate can be, for example, a sheet or non-woven material ofsynthetic fibers, a sheet of impermeable plastic material in whichperforations have been made according to a particular distribution. Thelayers which must be applied to the rear side of the first substrate, orat least some of them, can be applied before or after application ofthis first substrate already coated with front surface layers to thesecond substrate.

In another method of implementation of the present subvariant, thelayers which must be applied on the reverse side of the first substrate(or at least some of them) can be so applied after application of thisfirst substrate, already coated with rear side layers to the secondsubstrate. This latter method is particularly advantageous when thedevice comprises a multiplicity of electrical current leads to theworking electrodes, distributed over the surface of the secondsubstrate. It is easily possible (by application, for example, of asingle layer suitably distributed in appropriate portions forming anassociated pattern of electrical connection material such as aconductive ink or paste or equivalent on the reverse side of the firstsubstrate) to simultaneously fabricate the current leads in contact withthe working electrodes through the porosity of this first substrate, andto electrically connect them to the network of internal electricalconnections of the light-modulating device.

According to a third variant of the first method of manufacture, the twofirst variants are combined with each other. That is, there is appliedpart of the constitutive layers or coatings of the various patternsconcerned according to the first variant, and another part according tothe second variant. The first substrate of the first variant then has tobe considered as merged with the second substrate of the second variant.

The first method thus described implements the thick film techniqueswhich are known to the man skilled in the art of these techniques. Forthis reason, these techniques are not described again here, theinvention essentially consisting in applying these thick film techniquesto the modulating cells and devices.

A second possible method of manufacture of a light-modulating devicecomprising a multiplicity of picture elements, in particular applicableto the preferred structure, which lends itself to industrial manufactureat low cost price. The second method comprises applying (to a singletransparent substrate comprising a surface pattern of transparentelectrodes of a coordinated surface pattern of dots, segments or stripscomprising, associated in a single composite film) at least one layer ofthe present electrolytic material and one counter-electrode (forexample, pellicular).

This second method of manufacture of a light-modulating device (inparticular of a display device) comprises manufacturing (according toknown techniques, in particular, of extrusion, rolling, calendaring,coating, or equivalent) a film, sheet or composite strip formed of atleast one layer of electrolytic material applied to a counter-electrodein the form of a film, sheet, strip or equivalent and preferablyflexible, and possibly a layer of electrical connection material appliedto the outside face of the counter-electrode film. It also comprisescutting out from this film or composite sheet (using known techniques ofpunching, stamping and laser cutting in particular) of elements in theform of dots, segments or strips.

In this second manufacturing process, at least part of thelight-modulating device is produced:

by fabricating a single composite sheet or film formed from at least onelayer of the present electrolytic material and one counter-electrode infilm or sheet form, preferably flexible, to which the layer ofelectrolytic material is applied, and possibly a layer of electricalconnection material applied to the outer face of the counter-electrode;

by cutting in this film or composite sheet elements in the form of dots,segments or strips;

in distributing these elements according to a pattern on a transparentsubstrate provided with a coordinated surface pattern of transparentelectrodes, these patterns being in particular such that theirintersection determines the desired pattern of picture elements;

by fixing these elements on the substrate by contact or bypressure-sensitive adhesion;

and possibly by applying:

an associated pattern of contrasting or peripheral masking material;

associated patterns of current leads, connection conductors andinsulation layers constituting at least part of the network of internalconnections of the device connecting the working electrodes and thecounter-electrodes of the elementary cells to a connection zone or zonesor to a connector or connectors. These patterns are combined in such away that the conductors are insulated from each other and from thetransparent electrodes and counter-electrodes to which they do not haveto be connected;

associated patterns of layers or coating of protection, insulation,impregnation or potting materials;

the order of these coatings being able to be different from the orderdescribed above.

According to this second process, the counter-electrode in the form of asheet or film or thin equivalent can be any of the previously describedcounter-electrodes made from homogeneous material, composite material,of simple or complex structure, preferably flexible and deformablewithout damage, of thickness preferably between about ten microns and afew millimeters, more preferably between about ten and a few hundredmicrons. The layer of electrical connection material possibly applied tothe outer face, can, if necessary, constitute an electrical current leadfor the counter-electrode. The layer of electrolytic material preferablyhas the pressure-sensitive adhesion obtained in a preferred composition,and thus enables cohesion of the elementary cells of the device to beobtained by simple application to the substrate of the dots, segments orstrips cut from the composite film or sheet.

The second method thus described therefore implements the mentionedtechniques of extrusion, rolling, calendaring, coating or equivalent,laser cutting, punching, stamping, techniques which are known to the manskilled in the art of these techniques. For this reason, thesetechniques are not described again here, the invention essentiallyconsisting in applying these techniques to modulating devices and cells.

Finally, the second method enables production of composite films thusdescribed which are in the form of a film, sheet or strip comprising atleast one layer of electrolytic material applied to an electronicallyconductive film, sheet or strip, and, if necessary, a layer ofelectrical connection material applied to the outer face of theelectronically conductive film, sheet or strip.

EXAMPLE 1

A fluid formative composition is prepared as follows:

EXAMPLES

    ______________________________________                                        Zinc bromide           6.0 parts by weight                                    Copper chloride        0.1                                                    Calcium chloride       1.6                                                    Hydrochloric acid      0.2                                                    Triton X 100           0.2                                                    Hydroxyethylcellulose  1.6                                                    "Natrosol 250 HHRR" by "Hercules"                                             Water                 90.5                                                    ______________________________________                                    

This corresponds to the electrolytic material whose composition inequilibrium with a relative atmospheric humidity of 50 percent is:

    ______________________________________                                        Zinc bromide       43.2   parts by weight                                     Copper chloride    0.7                                                        Calcium chloride   11.5                                                       Hydrochloric acid  <0.1                                                       Triton X 100       1.4                                                        Hydroxyethylcellulose                                                                            11.5                                                       Water              31.6                                                       ______________________________________                                    

(where the ratio between the weight of water-soluble salts and that ofthe water is 1.75 and whose pH is approximately 1.7).

The above fluid formative composition is prepared by dissolving thefirst five constituents in half of the total water. To this is added,while stirring, the hydroxyethylcellulose previously dissolved in therest of the water. This fluid formative composition, whose pH is 1.6,has an appropriate viscosity for being coated with a coating bar. On asecond transparent electrode 2 of tin dioxide adhering to a glass plate13 ("NESA" glass by PPG INDUSTRIES) several layers are successivelyapplied, with intermediate dryings in hot air until solid consistency isobtained, until a layer of electrolytic material is obtained having atotal thickness of twenty microns. The extent of this layer, which istransparent and practically colorless, is limited to a disk 3 ofapproximately 1 cm². On the free face of this layer of electrolyticmaterial which exhibits a tack, there is applied a second transparentelectrode 4 of tin dioxide, itself also adhering to a glass plate 14 (asecond plate of "NESA" glass), having dimensions larger than those ofthe disk of electrolytic material (FIG. 10).

The transmission light-modulation cell 1 thus obtained (whose cohesionis provided by the adhesive properties of the electrolytic materiallayer alone) has (in the zone defined by the disk 3 of electrolyticmaterial) an optical transmission which is higher than in the peripheralzone which comprises only the two "NESA" glass plates 13, 14. Thishigher transmission is very certainly due to smaller losses byreflection at the interfaces of the electrode and electrolytic material,than at the interface between the electrode and the air.

There is applied between the transparent electrodes a potentialdifference of 2.5 volts. By transmission, there is observed aprogressive and uniform increase in the optical density of the cell overan area exactly delimited by the disk 3 of electrolytic material. Byreversing the polarity of the potential difference, a lightening of thedensified zone is observed until the restoration of the initialtransmission or transparency. When this stage is reached, it isnecessary to remove the erase voltage which would become a write voltageand start a new densification of the cell.

It is also possible to erase, but more slowly, simply byshort-circuiting the cell 1.

It is also possible to combine a partial erasure by a time-limitedapplication of a reverse voltage with the erasure of the residualoptical density by short-circuiting. The variable light-transmissioncell 1 thus obtained functions as a grey filter of which the opticaldensity can be varied continuously from the initial transmission ortransparency by controlling the duration for which the current is madeto flow.

It is observed that there is a voltage threshold of about 1.8 volts,below which the cell 1 is not densified. Such a threshold enables thecell 1 to be lightened with an erase voltage lower than this thresholdwhich can be maintained beyond complete erasure without fear ofrestarting an increase in the optical density despite this maintaining.

In a variant embodiment and constitution of the cell 1, there is applieda layer of electrolytic material on one of the transparent electrodes 2supported by its glass substrate 13, and a second layer of electrolyticmaterial on the second transparent electrode 4 supported by its glasssubstrate 14. The cell is formed by joining the two half-cells byapplication of the two free faces of the electrolytic material to eachother. The adherence of the two layers to each other suffices to providethe structural cohesion of the cell 1.

The same tests with the transparent electrodes 2, 4 comprising a layerof mixed tin and indium oxide adhering to a glass plate ("ITO coatedglass" by Donnelly) instead of the previous transparent electrodes isrepeated. The appearance and behavior are substantially similar.

EXAMPLE 2

The following fluid formative composition is prepared:

    ______________________________________                                        Zinc (II) bromide  6.0    parts by weight                                     Copper (III) chloride                                                                            0.1                                                        Calcium chloride   1.6                                                        Aluminum chloride  0.2                                                        Hydrochloric acid  0.2                                                        Triton X 100       0.2                                                        "7 HOF" Sodium     1.6                                                        carboxymethylcellulose                                                        by "Hercules"                                                                 Water              90.1                                                       ______________________________________                                    

This corresponds to the electrolytic material whose composition, inequilibrium with a relative atmospheric humidity of 50, is:

    ______________________________________                                        Zinc (II) bromide   42.9   parts by weight -Copper (II) chloride 0.7          Calcium chloride    11.4                                                      Aluminum chloride   1.4                                                       Hydrochloric acid   <0.1                                                      Triton X 100        1.4                                                       Sodium carboxymethylcellulose                                                                     11.4                                                      Water               30.7                                                      ______________________________________                                    

During drying of the fluid formative composition, the sodiumcarboxymethylcellulose is progressively cross-linked by the trivalentcation Al(III) and therefore becomes crosslinked in the constitutedlayer of electrolytic material. This layer adheres to the surface onwhich it is formed, but its free surface does not exhibitpressure-sensitive adhesion.

On two glass plates, each covered with a transparent electrode, there isapplied, as in Example 1, a layer of the above electrolytic material.Each layer adheres to the corresponding transparent electrode, but doesnot exhibit a tack on its free face. There is then applied on the freeface of one of these two layers a very thin layer of the fluid formativecomposition of Example 1, which is then dried. Then, the two halves ofthe cell are joined and the cohesion of the cell is then maintained bymeans of the pressure-sensitive adhesion of the last applied layer.

The properties of the transmission cell thus formed are substantiallythe same as those of Example 1, but its mechanical solidity is greater.

EXAMPLE 3

The fluid formative compositions and the electrolytic materials aremodified, starting from those described in Examples 1 and 2, in thefollowing way:

a) The following mixtures, expressed in relative weights of theirconstituents, are substituted for the mixtures of salts used in Examples1 and 2:

    ______________________________________                                                       Relative weights                                               ______________________________________                                        Variant 3.1.1                                                                 Zinc chloride    120                                                          Nickel chloride (II)                                                                           30                                                           Copper (II) chloride                                                                           1                                                            Variant 3.1.2                                                                 Cadmium nitrate  2                                                            Calcium chloride 1                                                            Variant 3.1.3                                                                 Zinc perchlorate 60                                                           Copper (II) perchlorate                                                                        1                                                            Nickel (II) chloride                                                                           30                                                           Variant 3.1.4                                                                 Zinc bromide     60                                                           Iron (III) chloride                                                                            1                                                            ______________________________________                                    

b) The following resins are substituted for the water-solublefilm-forming polymer resins used in Examples 1 and 2:

Variant 3.2.1

Polyvinyl alcohol ("Poval 224" by Kuraray)

Variant 3.2.2

Polyvinylpyrrolidone ("K 90" by GAF Corporation)

c) The ratio (weight of mixture of water-soluble salts)/(weight ofpolymer resin) used is modified by about 5 in Examples 1 and 2 to bringit to the values 50 and 0.5.

The substitutions and modifications thus mentioned can be combined witheach other. That is, the described variants can be combined.

With each of the combinations produced, variable light-transmissioncells are produced in the same way as in Examples 1 and 2.

EXAMPLE 4

The following fluid formative composition is prepared:

    ______________________________________                                        Zinc bromide         6.0    parts by weight                                   Copper (II) chloride 0.1                                                      Calcium chloride     1.6                                                      Hydrochloric acid    0.2                                                      Triton X 100         0.2                                                      Hydroxyethylcellulose                                                                              1.0                                                      "Natrosol 250 HHXB" by "Hercules"                                                                  16.0                                                     Titanium dioxide "RL75" by                                                    "Titafrance"                                                                  Water                74.9                                                     ______________________________________                                    

corresponding to the electrolytic material whose composition inequilibrium with a relative atmospheric humidity of 50 percent is:

    ______________________________________                                        Zinc bromide       20.0   parts by weight                                     Copper (II) chloride                                                                             0.3                                                        Calcium chloride   5.3                                                        Hydrochloric acid  <0.1                                                       Triton X 100       0.7                                                        Hydroxyethylcellulose                                                                            3.3                                                        Titanium dioxide   53.3                                                       Water              17.0                                                       ______________________________________                                    

Using a coating bar, successive layers are applied, followed by dryings,of the fluid formative composition to the transparent electrode 2 of aplate of "NESA" glass until a layer of electrolytic material 3 isobtained, corresponding to a total thickness of about 100 micronscovering the totality of the transparent electrode, with the exceptionof a peripheral band 16. This layer is white and opaque.

A disk of diameter 6 mm is cut in the following counter-electrodes 4,all in the form of flexible thin sheets:

Variant 4.1

Counter-electrodes in homogeneous material:

Sub-variant 4.1.1

Flexible pure graphite-sheet "Grafoil" by Union Carbide

Sub-variant 4.1.2

Pure graphite flexible sheet

"Papyex" by Carbone-Lorraine

Variant 4.2

Counter-electrodes of composite material:

Sub-variant 4.2.1

Sheet of plastic material filled with particulate carbon "Condulon" byPervel Industries

Variant 4.3

Composite counter-electrodes with conducting substrate:

Sub-variant 4.3.1

Graphite sheet "Papyex" covered with a layer of graphite-based silkscreening ink "Electrodag 423 SS" by Acheson.

Sub-variant 4.3.2

Graphite sheet "Papyex" covered with a layer of graphite-based silkscreening paste "TU20S" by Asahi Chemical.

Sub-variant 4.3.3

Graphite sheet "Papyex" covered with a layer of copper-based silkscreening paste "ACP-020J" by Asahi Chemical.

Sub-variant 4.3.4

Graphite sheet "Papyex" covered with a layer of conductive nickel-basedvarnish "Condulon 245" by Pervel Industries.

Variant 4.4

Composite counter-electrodes with insulating substrate:

Sub-variants 4.4.1, 4.4.2, 4.4.3 and 4.4.4 respectively identical tosub-variants 4.3.1, 4.3.2, 4.3.3 and 4.3.4, except that the flexiblegraphite sheet "Papyex" is replaced by a non-woven sheet of 30 micronthick polypropylene "Paratherm".

On the rear side of each of the counter-electrode disks, opposite theelectrolytic material, there is deposited a layer 17 of silver lacquer"200" by Demetron. These disks 4, 17 are then applied (with a spacebetween them) to the layer of electrolytic material 3. Finally, the rearside of each disk 4, 17 is connected to an edge 18 of the glass plate 15by means of a pressure-sensitive adhesive copper strip Bishop "EZ" 19resting on a pressure-sensitive adhesive polyester strip which insulatesit from the layer of electrolytic material 3 and from the transparentelectrode 2. This copper strip 19, which can easily be connected fromthe edge 18 of the plate to an external voltage source, is electricallyintegrated with the counter-electrode 4 by means of a drop 21 of silverlacquer "200". Finally, there is applied to the periphery of thetransparent electrode a peripheral string of silver lacquer 2 whichenables the transparent electrode 2 to be connected to the externalvoltage source. Neither the counter-electrodes nor the connections arevisible or perceptible through the white opaque layer of electrolyticmaterial (FIGS. 11 and 12a).

The disks 4, 17 thus applied have a certain adherence to the layer ofelectrolytic material 3. However, this adherence is variable from onedisk to another, and irregular from one point to another of a same disk(which, in operation, results in heterogeneities of optical density). Apressure is therefore applied to each disk in order to obtain andmaintain a satisfactory electrical contact.

A potential difference of 1.5 volts is then applied between eachdisk-shaped counter-electrode 4, 17 and the transparent electrode 2, thelatter being negatively polarized with respect to the counter-electrode4. There is observed by reflection a darkening of each cell 1 accordingto an area exactly delimited by the projection of the disk 4, 17constituting the counter-electrode. The optical density is uniforminside of each area, and it is possible to vary it according to acontinuous grey scale by modulating the duration for which theelectrical current is made to flow.

By applying a potential difference in the opposite direction, also of1.5 volts, the created optical density is erased and the initial whiteappearance is restored. It is observed that it is possible to prolongthe application of the erase voltage beyond the total erasure withoutvisible disadvantage. The density obtained for a same duration ofapplication of the write voltage varies according to thecounter-electrode, which indicates possible differences in impedance.

It is noted that all of the cells, with the exception of those formedwith the counter-electrodes of sub-variants 4.3.3, 4.3.4, 4.4.3 and4.4.4, have a write threshold of about 1.25 volts. That is, no darkeningis observed with a lower voltage. On the other hand, the cells formedwith the above-mentioned four counter-electrodes have no threshold.

EXAMPLE 5

The tests of Example 4 are repeated, but with application on the frontside (that is, the face intended to be in contact with the electrolyticmaterial) of each counter-electrode, before cutting the disks, a layerof about ten microns of electrolytic material of Example 1, according tothe procedure of this Example 1.

The disks thus coated are then applied to the layer of electrolyticmaterial of the Example 4 covering the transparent electrode on itsglass substrate. This time, the disks remain stuck. The cohesion of thecells is maintained without it being necessary to apply pressure.Similarly, without external applied pressure, the functioning of eachcell is uniform.

EXAMPLE 6

The tests of Example 5 are repeated with the following differences:

1) no layer of electrolytic material is applied to the transparentelectrode 2 fixed on its glass substrate 15;

2) on the front side of each counter-electrode 4 there is applied alayer of electrolytic material 3 of Example 4 in a thickness of about100 microns (according to the application procedure described in Example4);

3) from the composite sheets (counter-electrode--electrolytic material)thus produced composite disks 23 are cut out which are each applied tothe transparent electrode 2. In this case, unlike that which wasobserved in Example 4, and probably due to the smoothness of the surfaceof the transparent electrode 2 unlike the generally irregular surface ofthe sheets of counter-electrode material, the adherence is uniform andsatisfactory, despite the weaker pressure-sensitive adhesion of thismaterial than that of the electrolytic material of Example 1. Theindividual cells do not require the application of a pressure in orderto maintain their structural cohesion nor ensure good electricalcontacts (FIG. 13).

The functioning of each cell is comparable to that of the cells ofExample 5. The optical density is uniform inside the composite disk.

EXAMPLE 7

There is applied a layer of electrolytic material 3 of Example 4 to atransparent electrode 2 supported by a glass substrate 24. On this layer3 there is deposited by silk screening, portions of a layer in the formof square dots 25 sides 3.5 mm in length, of "Electrodag 423 SS"graphite based ink. On these square dots 25, after heat treatment, thereis also deposited by silk screening portions of a layer in the form ofsquare dots 26 sides 2.5 mm in length, of silver-based ink "429 SS" byAcheson for silk screen applications. Finally, again by silk screening,there is applied a layer of Acheson "432 SS" silk screening insulatingvarnish 27 covering the entire surface of the plate, except for thesilver ink square dots and a strip 28 at the periphery of thetransparent electrode 2.

There is deposited by gun spraying through a mask strings 29 of silverlacquer "200", connecting and joining each silver ink square dots 26 toan edge 29 of the glass plate 24 from which it is easier to make anelectrical connection with an external power supply. At the same timethere is deposited a peripheral band of silver lacquer, also enablingthe transparent electrode to be connected to the external supply (FIGS.14 and 15).

The appearance and characteristics of each cell are similar to those inExamples 4 and 5. In particular, the densification is uniform inside anarea which is exactly delimited by the orthogonal projection of thesquare dot 25 of graphite ink.

EXAMPLE 8

The procedure is as in Example 7, except that there is deposited on thetransparent electrode (instead of a continuous layer of the electrolyticmaterial concerned) portions of a layer of this material in the form ofsquare dots with 3.7 mm sides, by silk screening, the fluid formativecomposition of Example 4 having been brought, by evaporation of water,to an appropriate viscosity for this application. Furthermore, insteadof the transparent insulating varnish of Example 7, there is appliedaccording to the same geometry, making use of the same openings, a layerof a few hundred microns thick of white silicone elastomer "RhodorsilCAF 730" by Rhone-Poulenc, which vulcanizes in air in a few hours.

The appearance obtained is the same as that of the cells of Example 7,with a slight difference of color between the white of the square dotsof electrolytic material and that of the white silicone elastomer, adifference which does not exist in the devices of Example 7. Thedarkening takes place in an area which is the orthogonal projection ofthe square dots of graphite ink (and not the area of the portions ofelectrolytic material).

EXAMPLE 9

On a flexible sheet of 50 micron thick polyester, there is successivelyapplied with dryings and/or intermediate heat treatments:

1) portions of a layer of silver ink for silk screening in the form ofsquare dots with 2.5 mm sides, extended by a continuous strip to oneedge of the sheet;

2) a layer of insulating varnish with openings of 2.5 mm side length,corresponding to the previous square dots;

3) portions of a layer of graphite ink for silk screening in the form ofsquare dots of 3.5 mm width, centered on the silver ink square dots;

4) a layer of electrolytic material of the Example 4.

The flexible composite sheet thus obtained is deposited by unrolling iton a glass plate comprising a transparent electrode ("MESA" glass)provided with a peripheral strip providing contact with the transparentelectrode. The composite sheet overlaps the glass plate in order toenable access to the ends of the silver ink strings.

The composite sheet adheres to the "NESA" glass due topressure-sensitive adhesion of the electrolytic material, and structuralcohesion of the display panel thus produced is provided without othermeans.

Each cell is individually addressable by applying a voltage between thetransparent electrode and each silver ink string connecting eachcounter-electrode. The darkening of each dot appears exactly delimitedby the orthogonal projection of the area of the square of graphite ink.

According to a variant embodiment of the present device, the plasticsheet serving as a primary substrate is a sheet coated with ananti-adhesive layer having a very weak adherence to the layers depositedon it. After producing the panel, this sheet is withdrawn and hastherefore served only as a convenient intermediary in the production ofthe display panel.

EXAMPLE 10

On a flexible polyester sheet of 50 micron thickness, there issuccessively applied with intermediate dryings and/or heat treatments:

1) portions of a layer of silver ink for silk screening in the form ofparallel strips of width 0.3 mm;

2) strips of insulating varnish for silk screening, filling gaps betweenthe above strips of silver ink;

3) portions of a layer of graphite ink in the form of parallel strips ofwidth 0.4 mm, centered on the silver ink strips;

4) portions of a layer of electrolytic material of Example 4 in theshape of strips of width 0.45 mm, centered on the above strips.

The composite sheet thus obtained is applied, by unrolling it, onto aglass plate covered with a transparent conductive layer of tin dioxidepatterned as independent 3.5 mm parallel strips separated by a gap of0.1 mm diameter. The orientation of the composite sheet with respect tothe plate is such that the strips or transparent electrode areperpendicular to the strips of the composite sheet. The composite sheetoverlaps the glass plate such that the ends of its strips are accessiblefor connection to the addressing and control electronics.

The composite sheet adheres to the glass plate coated with itstransparent electrodes because of pressure-sensitive adhesion of theelectrolytic material. Structural cohesion of the display panel thusproduced is provided without other means.

The columns comprising strips of conductive electrodes are connected bytheir ends to addressing and control electronics to which are alsoconnected ends of the strips of silver ink comprising the lineconductors of the panel.

The writing of this matrix panel can be performed row after row. Awriting voltage is applied between, on the one hand, all of the selectedcolumns and, on the other hand, a given row. The selected pixels of theconsidered row are thus darkened without darkening the non-selectedpixels of the same row, nor those of the previous and following rows.This results from the high value of the voltage threshold with respectto the writing voltage, as well as from the favorable memorycharacteristics.

According to a variant embodiment of the present device, the plasticsheet used as a primary substrate is a sheet coated with a layer ofhydrophobic silicone resin having a low adherence to the layersdeposited on it. After producing the panel, this sheet is removed, andhas therefore served only as a convenient intermediary in the productionof the display panel.

EXAMPLE 11

This example relates to a direct addressed display panel intended for aseven-segment alphanumerical character (FIGS. 16, 17, 18, 19 and 20).

Such a panel enables one of the ten FIGS. 0 to 9, respectively, to beproduced from the seven linear juxtaposed segments 31A, 31B, 31C, 31D,31E, 31F, and 31G. This arrangement is known, and comprises foursegments distributed in twos end to end along two longitudinal parallellines, and three transverse segments respectively upper 31E, lower 31Fand middle 31G.

This panel comprises a front section facing the reader at the rearsection, a glass plate 32 and on the rear face of the latter, anelectronically conductive and transparent layer or thin coating (forexample, of tin oxide) forming the working electrode 2. In a firstvariant (FIGS. 18 and 19), portions of a layer of electrolytic material3 (such as that previously described) and portions of a layer ofcounter-electrode 4 (also such as previously described) aresuperimposed, facing each other, arranged to constitute the segments 31Ato 31G mentioned above. According to the general structure alreadydescribed above, the layer of electrolytic material 3 is in contact withthe working electrode 2, while a counter-electrode 4 is in contact withthe material 3. The different segments 31A to 31G are separated fromeach other by spaces 33 which can be very small (for example, aseparation of the order of 0.5 mm for a character of about 12 cm inheight). In a second variant (FIG. 11c), the layer of electrolyticmaterial 3 occupies almost all of the surface of the conductivetransparent layer of the working electrode 2 with the exception of aperipheral strip 34.

In these two described embodiments, there are provided filiformconductors of electricity forming current leads to the transparentelectrode 2 comprising a conductive ink or paste or equivalent aspreviously described. More precisely, and in the case of analphanumerical character with seven segments 31A to 31G, there areprovided three filiform or long thin conductors 35A, 35B, 35Csurrounding the segments 31A to 31G, respectively outside and inside, asshown in FIG. 16. This arrangement enables supply and distribution ofelectrical energy which is as regular and satisfactory as possible forthe different segments.

The method of producing the electrical connector material forming thefiliform or long thin conductors 35A, 35B, 35C (as well as theirassociation with the plate 32 from the side of the layer and forming theworking electrode 2) have already been previously described and are notdescribed again here.

Furthermore, conductors of electricity are provided in the form ofstrips 36, each belonging to and applied in contact with a segment 31Ato 31G and of similar form but of dimensions smaller than those of thesegments 31A to 31G, comprising a conductive ink or paste or equivalent(also previously described), forming a current lead to thecounter-electrodes 4 of the segments 31A to 316.

In fact, in general, in this type of application, the area available forapplication of this conductive material to the counter-electrode 4 islarger than that which is possible on the electrode 2. In addition, thestrips 35 are not visible from the front face of the panel. In thisapplication, which functions by reflection, and in the indicatedstructure, it is necessary to provide a masking material 37 constitutinga background for the panel. This masking material 37 is arranged in alayer, of course not hiding the filiform or long thin conductors 35A,35B, 35C and the portions of layer of material 3. Such a maskingmaterial 37 has also been described previously, and is not describedagain here. The masking material covers the edges of the portions oflayers of electrolytic material 3 and of the counter-electrodes 4 andpartially the rear side of the latter, contributing to cohesion of thepanel and providing insulation of the cells thus fabricated.

The means of electrical connection and of complementary mechanicalsupport of the panel comprise a rear plate 38 which is parallel to theglass plate 32, and which is able to be associated with the latterparticularly by its edges by a joining seal 39, particularly of siliconeelastomer. Such a seal 39 has capacity to absorb differences of movementbetween the rear plate 38 and the glass plate 32.

In an embodiment illustrated more particularly by FIG. 18, the plate 38is a printed circuit board. It comprises, at the right of a strip 36, ahole 40 in which can be engaged an electrically conductive jumper 41which, by its inner end 42 comes into electronic contact with the band36. By its external turned down edge 43, it is applied to the externalface 44 of the plate 38 in contact with a flat conductor 45 of the plate38 on which it can be maintained and electrically associated by means ofa conductive ink or paste or equivalent 46.

The various conductors 45 (seven in this case) corresponding to thevarious segments 31A to 31G are electrically associated with a plug-inconnector 47 located on an edge of the plate 38. This plug-in connector47 has eight positions, seven of which, 48, correspond to the sevencounter-electrodes 4 of the segments 31A to 31G via the conductors 45and the jumpers 41, and one, 49, which corresponds to the electrode 2via other jumpers 50, passing through other holes 51 to the right of thefiliform or long thin conductors 35A, 350, 35C. The jumpers 50 arearranged on a conductor 52 of the plate 38, this conductor 52 comprisingthe rear electrically conductive surface of this plate, with theexception of the conductors 45. The different conductors 45 and 52 areinsulated from each other.

According to another variant (FIG. 19), electrical connecting sleeves 53extending the conductors 45 and 52 are provided in the holes 40 and 51.In each of these sleeves, there is inserted a drop of electricalconnecting material 54 (such as a conductive ink or paste or equivalent)which comes into contact with the strips 36 or the conductors 35A, 35B,35C, and provides an electrical "weld" at these places.

The two variant embodiments shown in FIGS. 18 and 19 correspond to thethird variant shown in the previously described FIG. 8C.

The panel can also be produced according to the first variant of FIG.8A, also described (FIG. 20). For this purpose, the electrolyticmaterial forms a continuous layer 3 and no longer a divided layer intosegments. This also enables the layer of material 3 to form acontrasting background for the image segments 31A to 31G. Thecounter-electrodes 4 are produced as before, as are the current leadsand electrical connections with regard to the counter-electrodes 4. Inthis case, the contour of the counter-electrodes 4 defines the shape ofthe picture elements 31A to 31G.

The current leads of the Working electrode 2 are produced either bymeans of a filiform conductor surrounding the plate 32 on its insideface, at the periphery (that is, on the layer comprising the electrode 2and outside the layer of electrolytic material 3, that is, in theperipheral strip 34), or by dots. These dots are distributedappropriately to ensure an appropriate distribution of the electricalenergy.

According to a detail of embodiment shown in FIG. 21, it is possible toassociate with the current leads made from a first electronicallyconductive material 55 (such as a conductive ink or paste or equivalentas previously described) a strip or wire 56 made from a second moreconductive material (for example, a copper strip or wire). This secondmaterial 56, poorly resistive, is in electrical connection with theprevious current leads 55 by overlapping or partial potting by means ofan additional amount 57 of the first material.

EXAMPLE 12

In FIG. 22 there is shown another variant embodiment of a display panelcomprising several (in particular three on this occasion) characters58A, 58B, 58C, each having seven segments, each as described before.These three characters 58A, 58B, 58C therefore enable display of anumber between 000 and 999. The three characters 58A, 58B, 58C arejuxtaposed. The panel is divided into three zones 59A, 59B and 59C,respectively, on each of which there are three characters 58A, 55B and58 respectively.

For this purpose, the working electrode 2 and, in particular, theconductive layer on the glass plate 32 is itself distributed in threeelectrically distinct and juxtaposed portions. The portions areseparated from each other by a minimum space, such as 60, which can beas little as about ten micron wide. The layer of electrolytic material3, of white color, is distributed in portions of layers having theshape, size and location of the segments constituting each character58A, 58B, 58C. The space 61 outside the segments is covered with a layerof white masking and contrasting material.

In a first form of implementation, the counter-electrodes 4 of identicalsegments of the three characters 58A, 58B, 58C can be connected to eachother in parallel. Furthermore, in this same first form ofimplementation, each of the portions of a layer, as regards the workingelectrode 2, is separately fed. The panel thus fabricated comprises aplug-in connector 62 having ten outputs (namely seven outputs 63 for theseven counter-electrodes 4 of seven similar segments of the threecharacters 58A, 58B, 58C, and three outputs 64 for the three portions oftransparent working electrodes 2).

The number of electrical connections is limited, while allowing controlof the writing of the different characters in time sharing mode. Theparticular embodiment of each character can conform with that which hasbeen previously described in the case of a single character in Example11.

According to a variant embodiment (FIG. 22) the electrical current leadrelating to the electrode 2 can be made in the form of a plurality ofdots such as 65 instead of filiform and long thin conductors such asthose previously described in Example 11. These dots are distributedaround the characters 58A, 58B, 58C in order to ensure an electricalenergy distribution as appropriate as possible. Also, as a variant, themasking material is black instead of white and the points 65 are black.

As a variant (FIG. 23), the electrical distribution relating to theelectrode 2 can be produced not by dotted current leads such as 65 asjust described, but by longitudinal strips 66 arranged on the edges ofthe panel at a place where there is no electrolytic material 3. Thisvariant enables the various combinations of coordinated patterns whichwere previously described with reference to the diagrams of FIGS. 8. Inparticular, the layer of electrolytic material 3 can be continuous,which enables the presence of a masking material to be avoided.

A second form of implementation (not shown) essentially differs fromthat which has just been described by the fact that, on the one hand,the transparent conductive working electrode is common to all of thecharacters (that is, common to all of the elementary cells of thedevice). On the other hand, the working electrode and eachcounter-electrode are individually electrically connected either to oneor several connection zones situated on the rear side of the panelcomprising the glass substrate and the components of the devicepreviously mentioned, or to a connector integral with a printed circuitboard applied to the rear side of the panel, or to means of contactssituated to the right of the electrodes and supported by the frame ofthe device with which the display board is integrated.

In the considered case which comprises three characters, each havingseven segments, electrical connection with the external supply andcontrol electronics is produced by means of twenty-two independentconductors. In the case of using a printed circuit board, electricalconnection between each current lead and the corresponding conductor ofthe plate can be produced as described in Example 11.

EXAMPLE 13

Another exemplary embodiment is now described, relating to a matrixdisplay panel (FIGS. 24 to 27) enabling the display, for example, ofalphanumerical characters.

Such a panel comprises picture elements arranged at the intersections ofseveral rows 67 and several columns 68, juxtaposed and identical to eachother and thus comprising a matrix. The matrix enables, depending on thecontrol given, a writing of an alphanumerical character, sign or desiredfigure. In this example, the panel comprises a front glass plate 69, onthe rear face of which are produced transparent and electricallyconductive strips 70 constituting electrodes 2, each common to all ofthe cells of a same column 68. The strips 70 can be very close to eachother, separated by the gaps just necessary for avoiding electricalcontact between them, depending on the technique used.

In this example, the portions of layer of electrolytic material 3 andthe counter-electrodes 4 are superimposed, of the same shape, and of thesame dimensions. They are located at the position of each row 67 andcolumn 68 node, in order to determine each of the pixels, whose largestdimension in the direction of the rows is slightly less than the widthof the strips 70, being totally located to the right and inside of oneof these strips 70. On the rear side of each counter-electrode 4, acurrent lead is produced by means of a connection material alreadydescribed for such leads.

To each of the rows, an electrical conductor 71 (such as a copper strip)electrically connects the various counter-electrodes 4 of a same row 67to each other on their rear side. The association and the contactbetween this conductor 71 and the current leads of thecounter-electrodes 4 is produced by means of an additional amount 72 ofthe same overlapping or cladding material as the lead itself. Anelectrical insulator separates the conductor 71 from the otherconductive components of the panel in the gaps between the pictureelements.

In the form of embodiment in which the picture elements are of smallsize (for example, those of the pixels of a computer screen), it isgenerally sufficient to provide current leads for the working electrodes2 only at the edges of the strip 70.

In the form of embodiment shown in which the picture elements have alarge unit area, the generally modest conductivity of the workingelectrodes 2 can give rise to a non-uniform electrical feeding of theelementary cells. There is provided a mode of distribution of electricalcurrent to a strip 72 comprising current leads 73 arranged in sectionsof filiform lines parallel to the lines 67 and placed between thepicture elements while being insulated from the electrolytic materialand the counter-electrodes 4. As a variant, the leads can be distributedas dots. Furthermore, the current leads 473 of a same strip 70 areconnected to each other by means of a collector-conductor 74. This maybe, for example, a copper strip or string of silver lacquer arrangedalong the corresponding column 68 and connected to the leads 73 of thiscorresponding column by an additional amount 75 of this lead materialproducing the partial cladding or overlap. An electrical insulatorseparates the collector conductor 74 from all of the other conductivecomponents of the panel.

Furthermore, there is provided a masking material 76 in a layer coveringthe rear face of the glass plate 69 with the exception of the zones inwhich the electrolytic material is located, as well as the zonesreserved for electrical connections to the electrodes. This arrangementis such that the collector-conductors 74 are masked by the maskingmaterial 76 and can, without prejudice, be placed at any desiredposition, provided that the collector-conductor 74 of a column 68 is notin electrical contact with the current leads 73 of the adjacent column68.

For this purpose, and in default of intermediate electrical insulation,the length (in the direction of lines 67) of the leads 73 is sufficientwithout being excessive in order not to interfere with thecollector-conductor 74 of the adjacent column 68. In this embodiment,the electrical conductors 71 cross the collector-conductors 74 withoutelectrical contact (for example, by means of an intermediate insulator).In the case in which the collector-conductors 74 are sufficiently rigidcopper strips, there is no electrical contact with the conductors 71between each other, given that they are located in two distinct planesseparated from each other by a gap equal to the thickness of the cell,the air forming an insulator. In fact, the collector-conductors 74 areplaced in the vicinity of the plane of the rear face of the plate 69,while the electrical conductors 71 are placed in another separated planecorresponding to the free rear surface of the counter-electrodes 4.

Such a panel can also be associated, towards the rear, with a printedcircuit board 77 providing the electrical contacts with the conductors71 of each row 67 and the collector-conductors 74 of each column 68. Inthe case shown, in which the panel comprises five rows 67 and fivecolumns 68, the printed circuit panel 77 comprises ten conductors 78insulated from each other which can be connected to a lateral plug-inconnector 79 having ten positions as described in Example 11.Thereafter, by applying the required electrical voltage differencebetween an electrical conductor 71 and a collector-conductor 74 via theplug-in connector 79 and the conductors 78, the writing of the pictureelement corresponding to the intersection of the row and column 67 and68 is enabled, corresponding to these conductors 71 and 74. Furthermore,it is possible to apply the electrical writing voltage between a singleelectrical conductor 46 and several collector-conductors 74 or,conversely, between a single collector-conductor 74 and severalconductors 71.

This embodiment can itself be the subject of numerous variants.

First, the very size of the picture elements can vary from very small(such as the pixels of a computer screen) to very large (for example,the picture elements having a size in the order of a centimeter ormore), particularly in the case of a public information display panel.The variants then concern the coordination of the constitutive patternsof the panel. In the described embodiment, the working electrodes in theform of strip 70 are common to each column 68 and separated between thecolumns 68. The portions of the layer of electrolytic material 3 and thecounter-electrodes 4 are separate, each belonging to an elementary cell.

But, in other variant embodiments, it is possible to design othercombinations of pattern coordinations, as has been described withreference to FIGS. 8. The variants can also relate to internalelectrical connections of the display device, particularly in accordancewith that which was mentioned in the previous examples.

Finally, variants can relate to the very method of manufacture of such apanel as previously described.

What is claimed is:
 1. A process of modulation of light, by reflectionor transmission involving a light-modulating cell, the light-modulatingcell comprising 1) a first electrode, which is transparent orsubstantially transparent and electronically conducting; 2) a secondelectronically conducting electrode, spaced transversely from the firstelectrode; 3) a layer of material having ionic electroconductivity, incontact with and interposed between the first and second electrodes,comprising a homogeneous mixture of solid consistency comprising 3a) ahydrosoluble salt or a hydrosoluble mixture of salts of at least onemetal which can be cathodically deposited from an aqueous solution ofone of its simple or complex ions; 3b) at least one initiallyhydrosoluble film-forming polymer resin; 3c) water; and 3d) an auxiliaryredox couple; the constituents 3a, 3b, 3c, 3d being selected sop as toallow plastic or viscoelastic deformability; 4) means for ensuringmechanical and structural cohesion and integrity of the cell; 5) meansfor ensuring permanence of internal electrical contacts between thefirst electrode, the material and the second electrode; 6) spacer meansto maintain the first and second electrodes spaced transversely fromeach other; and 7) electrical connection zones on the first and secondelectrodes, the process comprising:1) during at least one write phase,applying to the first electrode an electrical voltage which is negativewith respect to a voltage of the second electrode, so that there iswritten at least one picture element as an increase in optical densityin an interface region between the electrodes and the layer of material;and 2) during at least one erase phase, subsequent to a write phase,conducting an electrical current between the electrodes, the currenthaving a direction opposite to that of an electrical current flowingduring the write phase, so that the previously written picture elementis erased, the increase in optical density being diminished ordisappearing; wherein the process may be repetitive and able to compriseseveral pairs of one write phase and one erase phase.
 2. The process ofmodulation of light according to claim 1, further comprising:maintainingthe written picture element for a certain duration following a writephase, applying no external potential difference between the firstelectrode and the second electrode, the optical densification producedduring the write phase remaining, at least in part, for at least acertain time.
 3. The process of modulation of light according to claim1, further comprising:maintaining the written picture element for acertain duration following a write phase, during which a writing voltageclose to an electromotive force of an elementary cell in the writtenstate is applied.
 4. The process of modulation of light according toclaim 1, further comprising:causing to flow between the electrodes anelectrical charge smaller than that necessary for maximum opticaldensity to create an optical densification of a picture element, whichdensity has an intermediate value between an optical density obtained inthe erase phase and a maximum density obtained in the written phase. 5.The process of modulation of light according to claim 1, furthercomprising:applying between the first and second electrodes a voltagedifference at least equal to an electrical voltage threshold, belowwhich threshold the picture element cannot be written.
 6. The process ofmodulation of light according to claim 1, further comprising:during atleast a part of the erase phase, applying to the first and secondelectrodes an electrical potential difference of opposite direction tothat applied during the previous write phase.
 7. The process ofmodulation of light according to claim 1, further comprising:during atleast part of the erase phase, producing an electrical short-circuitbetween the first and second electrodes.
 8. The process of modulation oflight according to claim 1, further comprising:during a write phase,developing an optical-density-forming metallic deposit, by cathodicreduction of metal ions; and during an erase phase, erasing anoptical-density-forming metal deposit, by anodic oxidation.